There are 4 tiers for you to choose. Proving Grounds is always available for free. Then, there are Silver and Gold Tiers, which offer greater rewards but you need ticket(s) to enter. You can start in any tier you want as long as you have the ticket for it. Finally, there's Infinity Tier, but you have to wait for it until the last week of the season; and it gives you the best prizes including the Infinity-Framed copy of current avatar!