By your question it seems you don't know about bitcoin well enough so please learn about it first, it will take very little time to gain basic knowledge about bitcoin.
I would like to suggest you some sites, guide, youtube videos, to learn:
A Guide for Cryptocurrency Newbies for Overcoming FOMO
learn me a bitcoin
Explain Crypto To COMPLETE Beginners: Coin Bureau Guide!! (youtube video)

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you can buy bitcoin by using exchanges there are centralized exchanges(needs sharing personal info and need to do KYC) and decentralized exchanges(do not need much info about you),

You can start your investment anytime but some reminders to remember.

Use your own money for investment because it will help you having better psychology in this volatile and uncertain market.
Don't invest if you can not afford losing your money.
Don't use any leverage for investment because with leverage, you will have risk of liquidation.
Invest in Bitcoin, forget about altcoins that will cause loss for your portfolio.

Invest with DCA strategy and with Bitcoin.
https://dcabtc.com/

Investing in cryptocurrencies is very risky, but you can invite good morning to bitcoins with much less risk.  The first thing you need to accept when investing in Bitcoin is that you must have a long-term investment mindset and you must have enough patience.  On the other hand you can follow DCA to further reduce your risk as you will buy bitcoins at different prices as you will not invest at one time and your chances of losing will be reduced. I think these are enough to invest in Bitcoin

People talk about Bitcoin and you get to hear about it all the time but have you asked them to give you the major tools to work with when you're in it completely? Sometimes we only hear about the positive part of it and we quickly run into it without getting to know how to grow our funds or the part of losing money.
Getting help from investors in this Forum is better than going in   just like that, the main reason why most of the people going into Bitcoin lose money too much is because they don't get the important info to help them in the crypto world.

Thanks for your advice, I will happy to take more informations that will make me grow

Excellent point, buddy, but occasionally make sure to proofread your posts before posting. These are some things that new members of the forum should be aware of when they join; they should also know how their activities will progress. The most crucial thing, though, is the forum's rules, as without them, it won't feel like a serious place for them. Bitcoin is a good project with a bright future, but we should also recognise that it is an investment that requires skill, and many people aren't properly introduced to it by their friends; some friends may have pique their interest in the forum by making them feel avaricious.

I said that because, if you look at a lot of the posts made by newbies, you'll notice that many of them don't ask questions about the forum rules; instead, they ask questions about bitcoin and how to help them if they're not ready to invest right now. Learning about bitcoin is not something we should jump into; instead, we need to learn everything thoroughly to ensure that we don't make any mistakes that could cost us the money we want to invest in it.

Thanks for the information, please what are rules in the forum

Welcome to the forum mate.
When Satoshi created Bitcoin he created this forum to talk about the Bitcoin system to understand it and if any error comes, to solve it. Now the forum is very wide and there are many rules and rank systems. There are ranks from newbie to legendary, and to grow your rank you need to earn merits. But the question is how to earn merits so there is a very simple way for this you can write a quality post that engages people and sparks information or news and when someone loves your post he or she will merit your post. This is a simple way of earning merits. Below I am giving you the links to the rules and the system before doing anything in this forum, just read these two links.
1 ›https://bitcointalk.org/index.php?topic=178608.0
2 ›https://bitcointalk.org/index.php?topic=703657.0

Welcome to the forum mate.
When Satoshi created Bitcoin he created this forum to talk about the Bitcoin system to understand it and if any error comes, to solve it. Now the forum is very wide and there are many rules and rank systems. There are ranks from newbie to legendary, and to grow your rank you need to earn merits. But the question is how to earn merits so there is a very simple way for this you can write a quality post that engages people and sparks information or news and when someone loves your post he or she will merit your post. This is a simple way of earning merits. Below I am giving you the links to the rules and the system before doing anything in this forum, just read these two links.
1 ›https://bitcointalk.org/index.php?topic=178608.0
2 ›https://bitcointalk.org/index.php?topic=703657.0

OP @Nwada has giving you some great links you can definitely learn the basics of Bitcoin but I would add you focus on your safety and security in/while using Bitcoin because it's easy to lose your Bitcoin if you drop your guard. To stay updated about Bitcoin here on the forum you can pay attention to the Bitcoin board on this forum
https://bitcointalk.org/index.php?board=1.0

Well, if you need a place then you have come to the right place before the great bull run comes and this has been our place all over the period of time.

You only have to keep reading all the forum rules and stuff and every discussion that you may be interested with to interact. This forum is full of knowledges, some wit as well and as I have said, it's all about continuous reading.

Welcome and enjoy.

Welcome to this my forum brother , yes truly betcoin is a good project actually and it will be in the future , for a good beginning you can use this forum by posting everyday or when you have time to do it just for fun . Your activities will rise every 15 days , you must upgrade your level , when you achieve full member you can start your real project by finding a signature compagne.. , or bounties , this forum offers you many chances and opportunities,  Good luck to you .

Bitcoin is not hard to understand, only those who can't do some research will fail to understand it's potentials, I read a lot about Bitcoin using google search engine, I don't even know about this forum and there was no one to ask a lot from at the time, but this forum makes it easier for beginner to understand Bitcoin and crypto, so OP I welcome you to this forum, if you are ready to learn you will gain a lot using this forum.

Many members have already gave you the best way to start your crypto journey but I will like to give you some warnings,  do not try to look for free ways to earn bitcoin, there is none, you can do some research on this and I bet that you will see some free websites promising free bitcoin if you do some things, the truth is they will scam you.

Do not allow PM from strangers, teaching anything, they have a reason why they PM you from the start and believe me it's not a good reason, before you take any steps at all, come on this forum and ask people, do not be in rush, wait for members feedback, this will safe you from.so many traps that scammers have set for beginners like you, I hope you enjoy your stay on here.

May your journey in this forum be fruitful. You may have been inspired by your friends to join this forum wanting to learn about Bitcoin. You have already provided link information about Bitcoin. Make sure you are interested in what you are doing. Keep a positive mindset and be persistent in your efforts. Remember that success does not happen overnight, but with effort and hard work you can achieve your goals. Ask questions as needed in this forum with willingness to learn. There may be a lot of technical terms and complex concepts, but don't hesitate to ask for clarification and take the time to understand. Also be respectful and courteous to other members, as fostering a positive and collaborative community will ultimately benefit everyone involved. While you engage with forums and learn more about crypto, consider doing your own research as well. Good luck on your bitcoin learning journey!