Legal marriages are typically governed by statutes in each individual state. From the The 2010 Florida Statutes online:

Notice that the Quakers felt it necessary to go to all the trouble to get their group added into the Florida Statutes.

If you are planning on just hanging out your shingle as a "Marrying Merlin" or "Marrying Merline", you might simply go the $79 Notary Public path to protect your clients from possible legal hassles. There is an online guide for Notaries "solemnizing a marriage."

You could seek ministerial credentials from fellow Pagans within a respected organization like the Covenant of the Goddess. 

You can get a free online ordination from the Universal Life Church Monastery or Universal Ministries or from the Spiritual Humanist Clergy. Alternatively, you can pay for an online ordination from the Universal Brotherhood. There are more of such sites popping up online and they appeal to more than just a Christian market. Some of these groups have incorporated, hold regular worship services, attained 501(c)(3) status from the IRS, and are thus entitled to ordain clergy. Others are rugged individualists who shun any association with any form of government.

780,000+ tax exempt organizations are listed in IRS Publication 78. Although a group can be missing for whatever reason, this list can be searched by visiting http://apps.irs.gov/app/pub78

Perhaps some Unitarian Universalist congregation might be willing to name you an "elder in communion;" however, they would probably ask you to join their membership and then hang around for a while first. The UU clergy, who see themselves as "real ministers," have advanced degrees, certifications, and a rather strong union. They tend to look askance at interlopers. They might be more comfortable if you use the term "Lay Minister."

So what is an Elder in Communion? Let us say it is, "She or he who is linked by affection, association, and loyalty to a particular church and who shares in the essential elements of its spiritual values." 

Be aware that in "performing a legal marriage" you are serving two roles: an officer of the State and a representative of a spiritual tradition. And people really do want meaningful rites of passage aligned with their particular values. Marriage laws vary, so be aware when you cross a state line. See more on marriage law.

Here is a sample of what a Handfasting Ritual might look like plus an adaptation of the Episcopalian rite. A legal Wiccan handfasting is an endeavor  Do be aware that a rehearsal will be helpful.

Florida also makes provision for Premarital Course Providers supplying instruction of not less than four hours.

Some advice from a Florida Clerk of the Court's Office for those officiating at handfastings:

• There is good tips for clergy in the Notary's Guide.
• The license is valid only in the state where it is issued.
• Write everything in black ink.
• A Florida ceremony must be recorded within 10 days after it was celebrated. Yes, it's still legal, but recorded counts for a lot.
• The date must be written as Month, Day, Year, with the month spelled out.
• List the city, town, or unincorporated area, where the ceremony was performed, and not the name of the coven or grove.
• Officiant sign the form. Print your name and title on the form. A title that a bureaucrat will recognize is helpful. Give complete mailing address and phone number of the person performing the ceremony. If later there are questions, this lets the clerk get an answer promptly, so the filing will be complete.
• Although both a HPS and HP preside and both say the blessing simultaneously, there is only one space for clergy on the form and only on field in the database in which the marriage will be recorded. The clerks will not know what to do if they get two names. The same thing can happen went the families are connected to two different Muggle clergy and they invite them both to participate in the ceremony.
• Get the signatures of two witnesses.