Here's how I usually explain it to people, but it's easier if done in person.
Get you bike elevated in a stand (or have a friend hold it by the seat if you don't have a stand). Turn the cranks with your left hand (slowly), and slowly start shifting through the gears. Start in one, and work you way one at a time to seven. Now, shift slowly back to one from seven. Pay attention to how well the bike shifts.
Does it upshift (to smaller cogs) easier, or does it downshift (to larger cogs) easier?
Usually, when the cable tension is off, it shifts better in one direction than the other. It generally won't have a ton of difficulty shifting in one direction, but it will hesitate and/or skip in the other.
Once you determine that, put it in a gear it "doesn't like". Looking at the rear of the bike/derailleur, turn the barrel adjuster (where the cable enters the rear of the derailleur) in the direction of the trouble 1/2 turn at a time until the clicking/skipping stops.
Now shift up and down through the gears and see if it's better. There's a chance you have screwed the barrel adjuster either too far or not far enough, so continue tweaking the barrel adjuster 1/4 or 1/2 turn at a time. If it starts hesitating in the opposite direction, that means you've screwed/unscrewed too much, and you need to turn the barrel adjuster a bit the other direction.
It'll probably take you a bit to get the hang of it, but it's not rocket science.
You are probably also at a point where you need to replace the cassette and chain, but I can't be for sure on that.