What would I need to really focus on or study if I decided to transition to an 18 series MOS from a noncombat/support MOS?

Rucking, lot's of rucking, running (a five mile run should be easy), land navigation, SUT, and the Ranger Handbook (your bible if you make it past SFAS). Don't think about it too hard, just go, and if you go, DON'T QUIT. The rest you will learn in the SFQC, on the job, or in the many advanced schools you will have the opportunity to attend. One more thing, if you make it onto the teams, ask for Ranger School ASAP. It will greatly benefit your career track, open doors that "don't exist", and the sooner you go after the SFQC the easier it will be.

SGT don't worry about being a badass, just don't quit and pay attention to your body. I will go ahead and tell you will hurt, you will hurt worse than you have ever hurt before. Just know it is not fun, take it one day at a time, one event at a time, one step at a time. I think it is a great decision and I wish the best of luck.

Everyone will say PT and rucking, but since you are asking the question and I assume have done at least a bit of homework, I'll go another way.
Think of yourself as a member of a team that must be reliable 100% of the time. Think like you are always the one navigating, thinking ahead, looking for a defensive position, responsible for evac... because an SF team member must be interchangeable with every other member. If you aren't, you are a liability. It is more than just pulling your weight. It is the distinct possibility that you will have to carry your weight, carry your teammate, and still accomplish a mission with little or no guidance.
It is a mindset that is hard to get into, especially if you spent any length of time conventional and become conditioned to expect others to cover their ground while you cover yours.