Bicycle Mechanics - gear problem.

hi im new here.

well here is my problem. i have a problem with the back gears. when i am in the 3rd shift on my left hand (i always keep on that one.)when im on the 1st gear on the right hand on the back tire its in 2nd gear but when i get up to 4th gear it goes in to 6th gear and when i go into 7th it goes in to 8th. btw i crashed in to some bushes about a week ago. well is there any way i can fix this my self i am kinda taped out of money for about a month or so. i live in canada. so if i take it in to a shop about how much will it cost. or do i need to buy new gears or what ever.

Most likely you just need to adjust the deraileur check this how-to by sheldon brown.
http://www.sheldonbrown.com/derailer-adjustment.html

And this one at ParkTool
http://www.parktool.com/repair_help/FAQrindx.shtml

If just happened after the crash could be a bent deraileur hanger, but most likely just out of adjustment.

Try this.

Stand your bike up vertically against something and look at your derailleur from the back. Your derailleur arm should be vertical. Your description of your shifting problem, especially after a crash, is often the result of a bent derailleur hanger.

Assuming I'm right, you have a few options.
1. You can try to bend it back by hand and eyeball it into vertical. This often works and it's cheap, but it's imprecise at best and at worst, it can totally ruin your frame.
2. You can have a shop bend it back. They should have a tool that gauges the allignment so it should be more accurate.
3. Many aluminum frames have a replaceable bolt on derailleur hanger. If yours does, buy two. Finding the exact replacement that you need when you want it can be a frustrating process.