Under Florida law the standard is that you have never been adjudicated guilty of any charge in your lifetime. If the charge was dismissed, it can be expunged. If you entered a plea and received a withhold of adjudication, it can be sealed and then expunged after 10 yrs. of being sealed.
You first need to identify anything that may be on your record and try to gather as much info as possible, including (especially) case numbers. Remember that you can expunge any felony that you didn't go to the state prison for, and a number of them can be reduced to misdemeanors at the same time, depending on what the conviction was for.
Many people find having an attorney is quite helpful in this process. It's relatively inexpensive and can save a lot of hassle for you. If you want to try to figure out how to do it on your own, what you're looking for is the forms for a P.C. sec. 1203.4 motion (a separate motion for each conviction.) You can find the forms online and there's a filing fee when you go to file them.
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I am not sure from your question if you are eligible for relief under Penal Code 1203.4. What was the sentence on this last charge?
Was it imposed? Was there any violations of probation? Did you successfully complete probation? Is the charge a wobbler? It is hard to answer your question without more information from you or your attorney.