If anyone hasn't done one of the 3 round tournaments yet you should give it a try but essentially you have 8 players in a single-loss elimination bracket. Before the matches start you're all put into a chat room together and shown the full bracket. You get 3 minutes before the first match (I think) and 5 minutes before the next two rounds (or more if you win early).
Don't rush into the match! This 3 minutes is a lot of time to scout your opponent. You can right click their name and view their profile. Look up their 1v1 history and check as many of their prior matches as you can. Pay careful attention to their build order. Do they pool first? CC before rax? Early forge with a lot of pylons?
In the lower leagues and in a single-loss elimination format most people open with their go-to build. If your opponent likes to open with cheese or is overly greedy you can see this before you ever play them. A little bit of knowledge goes a long way.
Also if you make it to the 2nd/3rd rounds don't hesitate to ask prior opponents what their games were like. After someone loses in a tournament they aren't immediately removed from the tournament chat. If they stick around try to message them and ask for tips.