You never have to spend a dime on this game to unlock anything.
To unlock ranked, you need to be level 30+ with atleast 20 champions.
Normal draft you need at least 20 champions, but champions in the free rotation count towards that number.
You unlock champions by leveling up your account and getting champion shards. You can either use pay full price for a champion from the shot, use blue essence with a champion shard to unlock a champion, or you can disenchant a shard to get more blue essence. Unlocking champions through champion shards gets you a 40% discount over just buying them outright from the in game store.
Skins can be obtained by gettings S- or higher at the end of a game, which will get you a hextech chest. Hextech chests contain skin shards which you can upgrade into permanent skins by either using orange essence or rerolling 3 shards. You get orange essence by disenchanting skin shards.
You need a hextech key to unlock hextech chests, which are obtained through honors at the end of a game.
You're limited to 1 hextech chest per champion per season, and unlock 1 additional chest per week.
I would suggest against playing ranked immediately after hitting 30 so you can get a better feel for the game first, otherwise, you'll likely place pretty low, which makes climbing to higher ranks more difficult.
If you need clarification on anything, feel free to ask. It can be a bit overwhelming to new players.
[slayer-jinx-catface]