Learn To Read Music
If you don't already know how to read music, there are lots of resources that can help you. You will have to buy some teaching material for beginners at a music store or order online. You might even be able to learn using free online helps, but I recommend buying books. Go through the books learning all the beginning basics of music, but move on as soon as you can to a hymn book. You can also buy some note learning flashcards. These cards show notes on a staff on one side and note name and piano key location on the other side.
Being able to read music is essential for hymn playing, but you do not have to be expert at this. Before playing the hymn you must read all the notes to determine what chords to use and write the chord symbols. The top note has the melody of the hymn. In actual playing you will only be reading the top note in the hymn and the chord symbols. This would be equivalent to reading a lead sheet. A lead sheet is just a single line of notes with chord symbols.
You will need to learn the basics of music before you start reading and playing hymns.
While you are going through the beginner books you should also start to learn the scales, chord construction, and chord symbols presented here. And begin to practice playing the chords.