I'm a newbie to cycling and about as beginner as you can get.

My truck died a little over a week ago. I only live about 6 miles away from work (and can't afford a new vehicle anyway) so I decided why not start biking. I'm actually excited about it because I need the exercise (and to lose weight) and am looking forward to the commute.

I've got a $5 "yard sale special" mountain bike that I've used to ride around with the kids once in a while. Up to this point, aside from my kid years, I haven't been a cyclist. I've made the 6 mile ride with this bike 3 times and realize it's not going to cut it. So I've done some research and it seems that the GMC Denali is a good starter bike for the price so I've bought one (which I'll get this Friday).

I'm going to need to tote things on my bike, the most important being my laptop. I bought a laptop backback that I had planned on bungy-ing down on to a rack. I tried putting the rack on my bike I had last night and realized it needed 2 holes in the frame that weren't there, so I had to drill them. I don't plan on drilling on my new bike, so how do I pick a rack that will fit my new bike?

I've read a lot of the other threads on here, but any other general advice you would give a beginner like me?