I've hosted a LOT of bridal showers and bridal luncheons.
If it's the bridal luncheon (held the day before the wedding usually), you include the wedding party and immediate family of the bride and groom (moms, aunts, grandmas, etc.) The list of guests does come from the bride, but the guests will be from the above selection of people.
It it's a shower, which will probably be one of several, you get a list from the bride of who to invite. She will know how many showers are being given for her and will divide up the guests accordingly so that most people are not invited to more than one shower. This rule doesn't always apply though because there will be crossover guests and sometimes a person may be invited to more than one shower. A recent wedding I was involved in had all sorts of intertwined relationships for me.... my son's best friend was marrying the daughter of my best friend. My son was a groomsman, and I was the best friend of the mother of the bride. I attended 4 showers and hosted the luncheon. Whew!
So, once you've established a date, and gotten a guest list from the bride, you need to focus on getting the invitations printed and mailed at least 3 to 4 weeks before the shower.
What you do at the actual shower is really up to you. I prefer to have a nice spread of food including hors d'oeuvres, little sandwhich type things, dips, sweets and a cake with the bride and groom's names on it. You can have a sit down type event, or let people "graze" and sit wherever they want. After the food, let the bride open her gifts.
I know some people play shower "games", but I hate those things. They are horrible in my opinion, and I never have them at any showers that I host. Honestly, just socializing, eating and opening gifts is enough!
As to who helps with the shower, there is usually more than one person involved in the hosting duties. That includes food preparation, setting up, planning and shouldering the cost. It doesn't matter who you have helping you, but they will usually ask you if they can assist with the shower. It helps to have more than one person doing all the work and helping with the cost, but don't get too many involved or you'll have problems making decisions everyone is happy with. Get the bride's input on things... does she want the games? what are her wedding colors so you can decorate appropriately, what are her favorite foods and flavors, etc. Also, don't forget to list the hostesses on the invitation.
Good luck!