What must I do to get a token?

You need to farm more. For a 20 min game your farm is poor

You need to earn at least an S.

http://na.leagueoflegends.com/en/news/game-updates/features/champion-mastery-levels-6-and-7-live-now

Earn a S- or higher (on summoner's rift only I think) to get a mastery token, combine 2 of those tokens with a champion shard (there are other options, see link) to get level 6.

For level 7 you need to earn S ranks to get tokens, combine 3 of those in the same manner.

This is 8 month old info though, I don't know if anything has changes in the mean time.

Also, nobody knows what is required of you to gain those high ranks, don't believe people when they say you need to do this or that.

Don't die, and participate in a ton of fights.