Learning and practising go hand in hand. Practice is something you do all the time, not just at rehearsals.
Sing a song over and over in the shower, in the car, to yourself on the bus – wherever you can.
Practise singing without the words in front of you – the shower and the car are good places to enforce this! Learn a verse at a time. Once you know it, move on to the next – don’t waste time repeating the parts you already know.
If you have a portable recorder or if you know how to record on your phone, record the song during rehearsals, then practise at home with it.
When you come to rehearsal, you should have a good grip on the song and be ready to polish it with the rest of the group.