First of all I would like to say that you MUST gain enough knowledge to start trading.
To gain knowledge, you can watch yt videos, forums, blogs and also you can keep your eyes on chats of the trading platforms.

Only in chats, you will learn about the KINDs of traders we have in trading.

Traders write something like.... xxCoin to the moon, xxxCoin to the jupiter, WOW that coin is pumping "buy now" you will be rich, etc etc.
So basically a new trader will think, "yes" this & that coins price will increase soon. Lets buy it... But the real scenario is different..
The main thing is, you have to analyze every coin by yourself before investing.

So to start trading you can buy bitcoin using your own money
or you can earn bitcoin and then start trading.

There is lots of free ways to earn bitcoin.
Such as,
faucets (see micro earning section)
Sig. campaign (see service section)
Social campaign (see service section)
Other jobs according to your skills (see service section)

Patience needed both in trading and in free earning. So you MUST have it.

List of some good trading platforms.
https://poloniex.com
https://btc-e.com
https://yobit.net
https://www.livecoin.net
https://www.cryptopia.co.nz
https://www.bitfinex.com
https://www.itbit.com
https://www.bitstamp.net
https://btcmarkets.net
https://bleutrade.com
https://bittrex.com
https://novaexchange.com

Just to encourage you, I have started my bitcoin journey as a free earner.
Now I have some coins in bulk quantity... Though I am still trying hard to learn trading. So that one day I can be a PRO trader.

Good Luck OP!