HP,
I'd google or check the yellow pages for small, independent labs in your area -- I recently started shooting medium format and found a nice place with good rates that was convenient to my office. If you are looking to work with images digitally, get quotes for processing to develop only for C-41 (color), Black and White, and E6 (Color slide film -- gives you a positive image instead of a negative). Then look into a reasonably priced flatbed scanner with support for 6cm negatives -- the Epson 4490 gets good marks here, and is around $180 ($150 after mail-in rebate). I actually just ordered one, but don't have it yet to show example scans. I had been working on a friend's Nikon Super Coolscan 8000 before that, but it's way out of my price range, and I wanted something at home to work with scans for web resolution.
Film can vary in prices, and you can do different things with different types. True black and white can be "push processed," allowing you to shoot at a higher speed than it is rated for. Ilford Delta 3200 is an example of a very high-speed film that acheives its 3200 rating by being pushed "one-stop" -- natively it is around 1600 iso.
For an idea on the different types of film you can get, take a look here --
http://www.bhphotovideo.com/c/category/7224/Film_120_220_Sheet_Film.html
You'll want to look at items in the "120 & 220" links. Stick with 120 for the Holga, it won't accept 220 rolls (220 is the same size vertically, but has a much longer strip on the roll).
If you have access to a darkroom, black and white can be developed easily if you are patient and willing to take the time to learn. Using a daylight tank can be a little more difficult for a beginner, as you are required to load it in the dark (if you have a closet that fits the bill that would be nice, but most folks are stuck using a glove bag, which is, in the words of Run DMC, tricky for a beginner).
You mentioned you were in HS -- is there a photo club, or perhaps a class room with a darkroom? We had one in my school, but that was ten years ago.
Hope this helps...
Jim