I want to trade altcoin but I don't know the very coin that will exist and have future. I need guildance to take this step
That would be tough. You need to build your own list on your portfolio based on your DYOR and not just by following other's portfolios.
Well then, for a start, try to pick some top performer coins or tokens daily and weekly on the Binance platform. From there, refer to the coin's community and lurk on their group, if any, to somehow see the inside stuff of the projects and how's the community talk about it.
It is just a start then after quite some time, you will be used to it and finally, you can now able to create your own way on how to pick your preferred alts.
In addition to reading the website and methods mentioned in the historical replies, I think there are two more things to have, a good mentality, many altcoin projects will choose to run away and disappear, and some require you to spend a lot of time to look at the shanzhai. The currency market, their volatility and volatility range are larger than that of Bitcoin. In addition, you have to be more accurate than a bulging wallet. Trading altcoins requires a good tuition fee.