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
If you are looking for an exchange, you can choose one of the several exchanges
listed on this page. However, keep in mind, exchange wallets are not for long-term holding of tokens/coins, if you want to hold them, please store them in your own wallet.
You should also understand, for some centralized exchanges, you have to meet KYC in order to trade (sell/buy/withdraw). So, make sure to read the terms and conditions of the exchange before registering and making a deposit.
Which altcoins have a future? It's a good idea to do some research on each token you find, don't buy because of the ranking in CMC or Coinecko, however, look at the development the team is doing.
If you don't want to do KYC for centralized exchange, you can use decentralized exchange using Uniswap, Pancake or other, just becareful when you are trying to connect or give approval for your tokens.