This question has probably already been answered, but, it's easy to enter a daily tournament, all you have to do, is battle durring the tournament, in a room that has a Golden cup next to the name.
now, ELO is harder, ELO tournaments last a whole week, and you can enter at any time, as long as you play with an ELO compatible deck. in addition to that, to qualify for a prize, you must participate in at least 5 ELO battles, and have more than 1000 points at the end of the week(your score starts at 1000, so it isn't hard to get that much)
now, the complicated tournaments are ones thrown by players.
you find out about them in the tournaments and events Forum.
joining one is a little more difficult, because you need to sign up, and you need to sign up before the tournament starts, and be prepared to participate.
did that answer your question? or did you have further interests?