Well, first of all you should get acquainted with how the system works.
The most important thing to remember:
Before you start working on a show, let the Channel Manager or Language Moderator know that you’d like to help out
This means that if the show has a Channel Manager (the little thumbnail on the left of the show's page, just below the title), then you have to ask for permission from the Tagalog moderator, if there is one, or the Channel Manager, if there is no Tagalog moderator. But it's better to find a show that has one, so that she or he can be of help.
If the show has no Channel Manager, or if the Channel Manager is viki (blue icon), then you can enter and subtitle freely. But please read the subtitling guidelines here before starting:
It takes some getting used to, especially the expected formatting for italics, breaks etc. But a good tip is to just select and copy paste the English, and then only change/translate the English text into your language, keeping the formatting intact.
If you want to try your hand before joining a team, a tip is to search for older films - before 1970. Many of those have no Channel Manager or moderator.
On the blue band over viki pages, towards the left, you have menu item "Explore". It's a search engine, and you can search by year too, to find older stuff.