Best service would be something local, but you can also check most used/trusted exchanges like Coinbase, Binance, Bitstamp. Be prepare to do KYC (Know Your Customer) procedure of proving your identity.
We have our exchange on forum, you can try to trade there, but do that only with trusted members/use escrow.
I just started studying the cryptocurrency system and now I'm learning a lot about it. If you advise reliable sources, where everything is described in detail and clearly for those who only want to buy cryptocurrency, I will be grateful.