Sumo tickets go on sale roughly about six weeks before the start of every tournament.
There are various categories and prices, as well as certain kinds of seats that are unique to particular venues, but generally speaking tickets fall into two main groups — masu (box) and isu (chair).
The former means sharing a roughly 1.3 square meter box with three other people. You'll need to sit cross-legged or seiza style to fit and even for smaller people it's a tight squeeze.