Selection Sunday is March 12, which is when all the teams will be announced. Between now and then, all the conference tournaments will be occurring, and the winner of each is guaranteed a spot. The smaller conferences and mid-majors, who are unlikely to advance as far, have those tournaments this weekend, and the major ones next weekend. The remaining spots, called at-large bids, won't be known until the 12th, but if someone is in the top 20 or so of rankings that someone mentioned above, they're almost positive to get in as well. Don't completely rely on the RPI rankings, though, as the committee takes other factors into consideration. Clear as mud?
tl;dr The teams are announced 3/12