What certifications can I do or classes can I take that will make me more desirable to get into the SF group?

There's no selection, it's not a competition, SF Group is just another airborne unit for a junior enlisted MI Soldier. You just reenlist to go there if there is an opening.

Do more PT.
On a serious note: Are you asking about just getting into the SF support group? If so, then its just a matter of requesting a transfer there. If you are asking about becoming an 18 series, you need "only" apply, then pass selection and the training...to which "do more PT" is likely good advice. Look at their min passing standards and train till you can far exceed them, then train some more.

I'm no expert but it is Special Forces, you have to be extremely fit as I suspect the job can be more physically and mentally demanding than most. Consider these areas for review: ACFT/APFT Scores, line score (could they be improved), are you airborne, do you have a DLAB on file, do you have an EIB badge, College?, have you considered ranger school, air assault, are you currently utilizing the exercise workout plan listed online, have you reviewed the army SF website resources, have you spoke to a recruiter? Just some ideas, also, I hear the JHATC course is pretty legit. I would look into that just for 35F career progression in general.

Are you wanting to stay MI or go 18 series? If the former is true, speak with those in your MOS and ask what you need to do to be the best 35F. If you want to do SF, they will train you to what they want. SFC Boyd really answered your question. If you want to be 18 series, look at what SF does and find your weaknesses. If you will do HAHO then consider jumping civilian if that worries you. Shooting, you can by an AR and live at a range. Look at your weaknesses.