Hi! Welcome here :)
You can join the tournaments during the signups phases. Usually, these start right after another tournament ends. For example, the tournament called "Rodan Cup" is currently in semifinals phase. When it will ends, the next tournament in the queue will start (Flying Blind, a BW OU tournament without the team preview).
You can join the tournaments during the signups phases. Usually, these start right after another tournament ends. For example, the tournament called "Rodan Cup" is currently in semifinals phase. When it will ends, the next tournament in the queue will start (Flying Blind, a BW OU tournament without the team preview).