Well, perhaps he will not go with the decentralized exchange because Op talking about a trading site that has a wide number of coins supported and I don't think decentralized exchange has that. Just like what recommended above, the Binance exchange has a large number of traders as of now and I think that is a perfect exchange to use. As long as you need not exceed the number minimum amount or else you will get asked to submit KYC.
^{[Bisq exchange]} is also good if you worried about private security.