What has already been said is real good advice, use what you have to start with. If you don't have a rifle or want a new one, a 6mm, 6.5mm, 7mm, or 30cal are all good choices. You dont really need a 7mm or 30cal for varmints, but for crop damage deer or such, the bullet selection in both is great. I started with a remington m700 .308win and that was an awsome gun, still is, but now it's a custom
For a starter gun, make sure it has good barrel life, because the number one most important thing is to PRACTICE. A 6mm BR, 6 Dasher or .243win would be good 6mm's, a .260 remington would also be good. 6mm BR or the 6 Dasher are hard if not impossible to find in a non custom gun (savage MAY make a 6 BR, not sure) but .243 win would be a good choice. For 6.5mm I would go with the .260 remington, better barrel life than .243win and same ballistics, but bullets cost a bit more (more cost per shot).
Check out 6mmbr.com and look at the cartridge guides on the left side menu, tons of info on all the calibers i mentions and more.
For 30cal, i would go with a .308win and be done with it. Great barrel life, easy to tune if you handload, and just all around good. Either Michael Eichelle or myself can help you out with loads and get it shooting like a 300 WSM. This will work for almost everything you could care to hunt, and awsome bullet selection. Can also always get a box of factory ammo if something happens to your handloads, if you loose your ammo with a 6mm BR, youe done for the day.
7mm's i would say arent exactly ideal for what your saying, a 180 Berger is a bada$$ bullet with a BC of .659, but they have less barrel life than the 30 cals, and really you dont need that kind of gun to start with. A .243 win or .260 rem will let you shoot varmints in the next county.
Hope this helped, if you need any info or just wana chat, im in highschool myself so share sort of the same point of view, and have the same kind of "i want this but i dont have money, how can i get close without breaking the bank?" problems. Just shoot me a pm.
Enjoy the site,