Since a commitment ceremony is not legally binding, really anyone that you want to can marry you. It may be a close friend or family member, or it may be a religious leader. (In Vermont, Justices of the Peace can perform a legally binding civil union.)
When considering asking a friend or family member to marry you, think about whether or not they make a comfortable public speaker. This person will have to stand in front of a crowd and speak clearly and easily. Consider people that you know who have had long and successful marriages or unions, who will often have wise things to say about the nature of love and marriage.
If you're looking for an ordained religious leader, speak with a minister or rabbi from your congregation. If you don't belong to a congregation, or your faith doesn't perform same-sex ceremonies, consider visiting your local Unitarian Universalist, Universal Life or Metropolitan Community church, which support GLBT weddings. Also, remember that several faiths leave the choice up to individual clergy, including Episcopalian, some other Protestant, Buddhist, and Reform Judaism. Consult GLBT publications, community centers and the internet for religious officiants who will perform commitment ceremonies.
This person should be available to you during the wedding planning
process to go over the order of service, what their remarks will be, how you will handle the vows (will you and your future spouse memorize them or will you follow a call and response format) and anything they will ask the congregation to do (stand or kneel for certain parts, join in singing, or jointly bless the union.)
If there isn't anyone who you feel is appropriate to perform the ceremony, consider doing so yourselves. In this scenario, you would welcome everyone and thank them for coming. You might say a few words about why having a public ceremony is important to you, or talk about how you met and the general history of your relationship. You would then turn to each other to say your vows, and to exchange rings. I recommend that you have everything memorized or be a good extemporaneous speaker, as you won't have anyone to tell you what to say, and note cards will look a little tacky.