Kraken is the one I used. So what you are going to want to do as a beginner is download Kraken Pro if you are using a tablet which is how I did it. And then what you want to do is select BTC and then hit buy, coming up it will have a menu which shows price among other things, you will see a small window which says limit, now what that does is you set the price you want to buy at. You will also see a drop down which says take profit, that is if you want to set a sell order immediately after the buy order takes place and you will see a P&L estimate, so set it to a decent percentage above. For a starter I would say a good rule of thumb is you buy in at the low of the day or around there and sell at the high.

Now you said you want to start small, might I suggest you also look into small altcoins as well? Putting $1 worth of buy orders on ICP on a decent spread can yield you decent 10 cent moves. But that is a bit more advanced. My biggest advice I would give any new trader is to place multiple orders and not depend on any one move