Have you tried reading what other users have already said?

In one sentence - there is nothing called "best" exchange. They are several exchanges and you to try a few of them to know which one is suitable enough for your use. Depending on which exchange you use the order placing procedure may be described in their Help documents but often it is very conspicuous even for a newbie.

You could also visit youtube to give you a "visual" impression of how the process is done but be careful with those videos because they are often promote shitcoins and low volume exchanges there because they are paid to promote them.