Chords: Learn how to play chords and chord progressions using good voice leading. Also learn about different and interesting ways to voice chords. Learn to read and improvise accompaniments using chord charts. Sometimes a chord sheet is all you get when learning a song. Most sheet music piano parts are just a piano accompaniment arranged to play solo behind a singer. This is not how a keyboard player plays in the context of a band.
Harmony: Learn about diatonic harmony in major and minor keys so you understand what makes a good chord progression work and can also analyze songs you want to know better. This will also give you the knowledge to be able to transpose easily. Also learn about such non-diatonic devices such as secondary dominants, parallel major and minor and modal interchange, all of which are used a lot in pop/rock music.
Bass lines: Learn how to play bass lines with your left hand. You won’t always need to do this but you will if there’s no bass player (Boo! I’m a bass player, can you tell?). Either way knowing what the bass does and how it works is important for knowing how to interact with it.
Improvising: Learn to improvise. Start simply, over a single chord or a blues. You will at some point be called on to play improvised solos.
Parts: listen to keyboard parts on some of your favorite records in different styles and try and emulate them. I’m sure there are tons of resources available on YouTube for learning previously recorded keyboard parts. This is really important because you have to be able to emulate established keyboard players to a point to learn how the keyboard functions in different pop styles and this will eventually lead to you being able to create your own parts that are relevant to what you are playing.
Blues: The blues is a huge part of the roots of pop/rock music. Spend some time researching blues piano styles such as boogie woogie, etc. Blues is also a great source for learning licks and phrases to help you with improvising and creating your own parts.
