The most popular exchange now is Binance and best trading coins are bitcoin and eth. You can consider top 20 cryptocurrencies from coinmarketcap. If you want to be daily trader, you should focus on top 20. But if you want to hold for long-term, you need more research and information to take a coin for 1-3 years. You also can trade on decentralized exchanges like IDEX, Forkdelta,... that in my opinion is still complicated for newbie.