After you figure out your budget, you need to talk with the mother to be in and get an idea of her guest list and the date. Then you can figure out a venue and your menu (ha).
You will need adequate seating, parking, bathrooms, and a place to prepare food. Churches, community center rooms, your home, or the MTB's home are all okay. If you have a lot of money a restaurant is another good option, but if you do this you will need to pay for everyone's meal.
Every shower I've been to has been in the mid-afternoon. You don't need to provide a full meal in that case, but I think it's nice to provide a wide variety of nibbles. At my baby shower, we had pigs in a blanket, meatballs, fruit and veggie trays with dips, chips and dip, cheese and crackers, chocolate fondue, pinwheel sandwiches, plus cake and punch.
Then there's the decor - a banner (front door, above the gift table, etc.), balloons, tablecloths, napkins/paper plates, some confetti (to sprinkle on the table), maybe a diaper cake (my friends made one out of cloth diapers and it was adorable), and some fresh flowers, for example. My friends made tissue paper poms that they hung from the tree in our front yard. The tree was blooming and was pink and white, and the tissue poms were blue and white, so it was just super cute.
Think about door prizes, favors, and games. My hostesses had a frame each guest could sign; we've got it hanging up in the baby's room and I love seeing it. There was a big plastic baby bottle filled with blue Hershey's kisses and each guest had to guess how many kisses were in there - winner got it all. There were also four games which I thought was one game too many. Find out what the MTB thinks - some hate games and don't want any, and others like me think games can be a lot of fun and make the shower flow better.
There are lots of ideas on Pinterest or just googling baby shower ideas. If you have another friend who's crafty or creative, she might be willing to help, and that can make your job a lot easier.