First of all you should feel so honored to be asked to be a bridesmaid, only the lucky few get chosen. This doesn't however mean that you can't respectfully decline. Some people just don't like the spotlight and your friend will understand. Let them know early though so they can organise a replacement because once your in- your in.
So what else are you in for?.....
Your in... for buying a dress
Some brides will pay for your gown however most of the time and as etiquette says this is at the bridesmaids cost. Within reason of course, your'd hope she's not going to ask you to fork out an excessive amount on a dress. She will also want your thoughts/input on the dress and for you be supportive. It's bad form to whinge because it's not exactly what you want but if your forking out for it, you should have some say.
Your in... to help plan and fund the Bachelorette party
It's the maid of honors duty to plan the bachelorette party but the bridesmaids are expected to put forward ideas and suggestions and offer assistance.
Your in... for attending the Rehearsal & Bachelorette Party
Unless your out of town or overseas you should really attend any wedding-related event. At the very least, the rehearsal which will be within a few days of the wedding so make sure you are in town a few days early.
Your in... for general wedding planning & possible production lines
You need to be there to help with decisions, listen to bitching and put up with bridezilla moments. If you can, help out with small duties and errands and basically make sure the bride is having a good time planning her big day.
Your in... for getting party started
You will be the glue that holds the dance floor together on the night, set a good example by getting out there and dancing up a storm.