I just acquired a used road bike, and while on my first ride I noticed that the bike gears were shifting by themselves. I dont know a whole lot about bikes so I looked up what kind of gear shifts it had, and they are down tube shifters. What can I do to stop the bike from shifting so much?

Are they indexed or friction?

Indexed (click between shifts) will probably require a simple adjustment of the rear derailleur cable. Go to Park tools website for help on that.

Friction shifters should just take some playing with the shifter to make sure it is centered on whatever gear you choose.

Either way, it doesn't sound like a big problem.

55/Rad

im guessing theyre friction, seeing as they dont click. so should i just ride it around and move the shifters until I get it to stop?

Yes. Also make sure that the nut holding the shifter to the frame isn't too loose. If so, the shifter will slip when riding.

You should b able to shift into all the rear gears and have the system hold it in any gear. If you have trouble getting into any gear and holding it, the cable and/or rear der may need adjustment.

What shifters and rear der do you have? How many speeds?

The shifters may be loose. Try tightening the bolt that attaches them. It may have a D-ring that makes this a simple finger-thumb adjustment.

i think 5

Tighten the thumb SCREWS.

Cable tension is only one possible problem (and perhaps the most common and easiest to fix). Other possible problems a worn out chain or worn out cassette or improperly installed rear wheel.

loose cassette, loose/misaligned rear drop out, stretched chain...

maybe you bought yourself one of these

Your bike is possessed. Call a church.

Any local bike shop should be able to show you how to fix that. Probably a good idea to have them give the bike a once over to check for other problems too. See if you can find a bicycle maintenance course near you.

Enjoy,

is it changing gears or is there a few stiff chain links?