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
there are so many potential altcoins today that are worth trading. You only need to look at CoinMarketCap which is currently ranked 20 and above, you only need to check Altcoins in terms of community, smart contract development and altcoin utility.
If your intention is to trade then that is fine you could even grab some coins outside the top 20 and inside the top 100 to trade, but only do so if you have a very solid exit strategy and you can follow it to the letter, as those coins can move incredibly quickly and there is nothing worst than to trade the coin and then the coin begins to go down and instead of selling it and accept a small loss you prefer to keep holding and then the losses that could be small become massive.