What exactly is the role of a Bridesmaid and a Best Man?

My bridesmaids will be 19, 18 and 15 and I honestly don't know what they're supposed to do other than look gorgeous on the day (and steady my nerves!!). Bit young to sort the hen do - and to be honest I'd prefer to sort it anyway.
The Best Man is my OH's stepdad - what, if anything should he be doing? I know he's on about organising their suits but other than that I don't know!!

Best man: organise the stag do, look after the rings on the day and provide general support, assist the ushers/groomsmen in shepherding guests from A to B if necessary, write and do a speech.

Bridesmaids: I think this varies a lot these days. For a lot of brides they just want them to look pretty on the day, and as you said yours are young so probably won't be organising the hen do. Other things on the day would include making sure your train and veil are sitting nicely at the back during the ceremony (mine are going to have great fun doing this, not!), looking after your belongings (eg phone, tissues, lippie, etc), and again just providing support and calming your nerves :-)

Thanks ladies - didn't want to offend them or make them feel unimportant by not giving them a job but Im sure they'll just be happy to look pretty!! :-) xx