Traditional responsibilities of a bridemaid include:

Helps the bride and maid of honor
Takes care of own dress and accessories fittings
May help with a shower for the bride (optional)
Assists the bride with errands
Contributes to bridesmaids' gift to the bride and usually gives an individual gift to the couple
Attends the rehearsal and rehearsal dinner
Arrives on time to get dressed for the wedding
Walks in processional and recessional
May participate in the receiving line
Dances with ushers and other guests
Helps gather guests for the first dance, cake cutting, and bouquet toss
Helps look after the couple's elderly relatives or friends
Pays for own dress and transportation to the wedding