What is the best exchange and how do I make a buy order there?
I suggest you visit
"Beginners & Help" first.
Making order is simple. Go to exchange find your pair (for example BTC/USDT, ETH/BTC, ...) and look at the current orders. Just make your shot and create your buy/sell order with price you want.
But about exchanges. Everyone has their own favorite exchange(s). I like Binance, Coinbase and Kucoin. But others Bitfinex, Huobi or Bithumb. There are more than 1000 exchanges on the market. So it is so hard to pick one best.