Start with your FH. You can have whomever you wish, there are no rules as to who can/cannot be in your bridal party. But I wouldn't choose too quickly. There are a ton of threads on here where people picked right away and then later had all sorts of issues and changed minds, but by then it was too late and they were stuck. I'd start first with other planning, and choose the party later on.
Proud Member of P.O.O.P., People Offended by Offended People
I agree, don't choose your wedding party so soon. Wait until about 9 months before the wedding to pick. Friendships change, people change, families can change. Anything is possible. You can make a list of who you may like to be in the wedding party and then when the time gets closer you can decide who to boot off the list and who to keep.
If nobody comes immediately to mind, there is no law that says that you must have a bridal party--you can skip it or have one attendant each. Most people turn to close friends and close relatives (including those of their future spouse). So, if you have a sister or sisters, close cousins, dear friends, his sister, these all are candidates. Some people even ask their parents to be their attendants. But, sometimes people have moved or are distant from family and friends. In this case, don't stress. Don't ask people just because you feel that you have to have a certain number of attendants. Whatever you do, avoid people who will only add drama and tension to your life. email@example.com
I agree with everything PP's have said. People and relationships do change, so I would wait 8 or 9 months out to choose your BP. Also keep in mind that the sides do not have to be even. If your FH decides he wants 4 GM's, it is totally fine for you to have 2 BM's. Also, guys can stand up on your side, and gals can stand up on his side. Don't ya love modern times!! I would like to second Myras in saying that your parents can even be attendants. My FH was having trouble choosing a best man because he has had a falling out with his brother, and his closest friend will be on his honeymoon at the time of our wedding. He then realized his dad was the closest person in his life, and that it would likely stay that way. FFIL was very honored to be asked.
Just remember that the only hard and fast rules/guidelines for your BP are in the way you treat your people. Be sure to choose those that are closest to you, and involve your FH equally in the selection. Don't make being a member of your BP into a job, and consider their limitations when choosing attire, accessories, and so on. And please do not decide to "fire" a member of the BP later on unless they do something drastically horrific (such as trying to sleep with the groom), and do not replace a member of the BP. The replacement will feel like a second class citizen, and the one who is replaced will likely no longer desire a friendship with you.