Congrat's on getting the ball rolling. Which program did you choose? Where you want to get a head start mostly depends on where you need some help the most. How are your Math skills? if you have a solid grip on intermediate Algebra then you're good. If not, maybe take some kind of online course. And, even if you're good at math, how about doing IV calculations and other types of problems you've never approached before. For lots of students in pharmacy tech programs, the math is what slows them down the most. Besides that, it might be good to start looking at abbreviations, sig codes, and Top 200 drugs. All that stuff is here on this website.
where to begin with learning by: Tracy
I would begin learning drug brand names, and their generic equivalent(s), their uses, contra-indications and adverse effects. There's hundreds of them, so making flash cards of drug names is a great start!