Technically speaking, your only "jobs" are to get the dress, attend the rehearsal (unless unavoidable job/family obligations prevent you from doing so), and show up to the wedding on time - dressed in the dress, clean and neat and smiling. In a perfect world, this is all the bride will expect from you, and in a perfect world it wouldn't be necessary for you to tell her that you're unable to do more than that.
Now ... some brides may "expect" more from their bridesmaids. Technically you are not obligated to do anything besides what I mentioned above, but some brides think that bridesmaids need to help with the wedding, attend pre-wedding events, fork over cash for pre-wedding events or pro hair/makeup for the wedding, or be happy little cheerleaders who never tire of hearing about their latest Pinterest projects.
And there are some brides out there who are truly not bridezillas who expect their bridesmaids to be their unpaid wedding planners ... but some brides may feel disappointed or offended if their bridesmaids don't offer to help with the plans or go on shopping trips or help plan a shower/bachelorette.
The shower and the bachelorette party are not mandatory. No bride is "entitled" to receive them - they're OPTIONAL gifts that the bridesmaids (or other friends) MIGHT choose to offer the bride. And if nobody offers them, then the bride doesn't get to throw her own. Not politely, anyway ... many brides see no problem with being rude enough to throw themselves parties where people are expected to show up with gifts for her or to pay her way at a bar or on a bachelorette trip or whatever.
Sometimes you'll run into a Maid of Honor or another bridesmaid who thinks that every bridesmaid must hand over a blank check to pay for pre-wedding parties, or that it's her job to boss the other ladies around. As a bridesmaid, you're absolutely free to say "Sorry but I cannot do that" if she demands something you don't want to offer. If she gets mad, oh well, too bad for her.
Long story short ... your best bet is to sit down with the bride and explain to her that you're honored to be asked to be her bridesmaid and you are happy she's getting married, but you have some questions about her expectations. Explain that you're happy to attend any pre-wedding events that the MOH/whomever might organize for her, but you'll be unavailable to help with planning those or helping to plan the wedding. It doesn't mean you're uninterested, it means you simply have your own life obligations to attend to.
If the bride is a reasonable person and says "No problem!" then you're all set. If the MOH/whomever approaches you about planning a shower or bachelorette, then it's best to tell her upfront EXACTLY what you're willing to offer in terms of finances and helping with the plans. If she doesn't like it, simply say, "Sorry but that's what I can offer you." If she still won't budge, then tell her that you'll happily attend as a guest and she can remove your name as a hostess.
If the bride gets mad at you for not being more available for her, though, then it's best to tell her, "In that case, it's probably best that I bow out of the wedding party and attend as a guest, since I cannot fulfill what you want from me." (And, honestly, I'd rethink the friendship if she'd rather you bow out than just simply be there by her side as she gets married. You don't need "friends" like that.)
The mere fact that you're even asking this question in the first place makes me think that you suspect that the bride won't be happy if you aren't her 24/7 happy helper. Right? If that's the case, then maybe you should rethink a friendship with someone who is only interested in you if you can do stuff for her.