Some brides require more help than others.
Basically, you should just be there for her ? go to dress fittings, help and support her when she chooses flowers, invitations etc, have her cry on your shoulder when she fights with her mother and so on.
You should also organise her kitchen tea/ hen's night including arranging presents, food, drinks.
On the day of the wedding you should help her dress, keep her calm, hold the bouquet when the rings are exchanged and see that things run smoothly.
Being a bridesmaid is often a thankless task. Be prepared to be supportive, kind and patient.
But don't let her walk all over you!