If you want to contribute with only English, there are two things:
Caption. Write the English subtitles when it’s an English-language movie or series, or if there are hard-subs printed on the copy.
Edit. When the Korean-English or Chinese-English subbers have finished their work, then the Translation Editor comes to check whether the translation is right, and then is the turn of the English Editor, who takes care of the English spelling, grammar and syntax, as well of continuity (of spelling, titles, nicknames etc).
You could also do a third thing, which has nothing to do with English:
Segment. Create, on the video timeline, the specific placeholders where the subtitles are going to go so that they are synchronized with the speech. This is a specialized skill, you have to undergo training (it typically takes from 2 to 3 months), but the trainers are few and overworked, so the waiting list is long.
Since it’s your first time, I strongly suggest you look at older shows, as moderators are more likely to hire an unknown person. There’s another reason as well, if you don’t have vikipass, you won’t even be able to access on-air shows. When you will have completed 3000 subs, you will be a Qualified Contributor and you will get a free vikipass, enabling you to access the majority of shows.
So find a show which has finished airing but is not yet completely subbed, look at the cover page (that ridiculously long image on the drama’s main page) where they list the team members and see whether they have an English editor or not. Then write a p.m. to the moderator. If the moderator’s name is not a link, note that the format is
You’re more likely to find work in a Chinese or Taiwanese drama because they are not as popular as the Korean ones.
Ah, and you may want to add your name to the English editor thread I just created today.
https://discussions.viki.com/t/english-editors-thread/23078. I included some useful resources too.