Bridesmaid

A bridesmaid is an attendant to the bride and a member of the bridal party.

Responsibilities

Selecting Bridesmaids

Beforehand, do not feel guilty about your final decision. Not all of your female friends/siblings/cousins could become a part of your wedding party.
If your fiancé has a sister, you may want to ask her.
Make sure to think long and hard about your choices and do not inform any of them until you are confident in your decision.
A bride may also choose to opt for a male bridesmaid.

Tips for Bridesmaids

It can be helpful for bridesmaids to carry an emergency kit on the day of the wedding for any mishaps that could happen on the day of. This kit can include: detergent wipes, a sewing kit, a small snack, breath mints, safety pins, white chalk, band-aids, tampons/pads, makeup or any other item that the bridal party could possibly need.
Brides are stressed about pretty much everything on their big day. They need you there as their bridesmaid to help them remember things... like eating and hydrating. Something you can help as a bridesmaid is to put aside some snacks for the bride, save an after-wedding meal for the new couple. They will LOVE you for thinking of them.
Be patient with a bride. She really doesn't intentionally want you to wear a hideous dress. She doesn't want to make you spend more than you have to on an ensemble. Communicate your concerns if you have any, otherwise, she will never know. If you aren't relaxed, how is she supposed to be?