During the beginning days, after you enter the trading world, just dig a lot of information and get enough knowledge about every aspect that has to be considered before choosing a coin to trade with. Get experience as you get to do more trades and learn from any mistake you make.
For trading, there is no fixed or average amount that is needed for starting it could be 100$ or even 1000$ depends on how much that person wants to keep the initial invested amount for trading.