What is the best exchange?

There are many exchanges in the world but the best/top exchanges recommended to use are:
1. Binance (number 1 bitcoin exchange)
2. BitMex
3. Bittrex
4. KuCoin
5. Kraken
6. Coinbase
7. Poloniex

You can see the detail information here: https://coinsutra.com/best-bitcoin-exchanges/

http://Actually, you are no need to ask, you can googling and find it out yourself, buddy.

-snip- how do I make a buy order there?

If you have already verified your account in exchange and you have deposited some money (USD), then you can place "a buy order" there.

Some of this stuff is more complicated than just Googling it, you idiot.