The blue/orange gels for basic colour-temp correction see less use, given most digital cameras now can set a white balance manually.
Gels are more useful for matching and adjusting light sources than doing the camera's white balancing. Even when shooting on film, it would be easier to change from a daylight to a tungsten stock than gelling every light source.
To the OP: have a look on Google for "colour temperature" and read up on it a bit. It'll help you begin to understand when and why lighting gels are used when lighting a film.
If you just want to buy a few gels and have a play, I'd recommend getting a few sheets of CTB (Colour to Blue) in various strengths (1/4, 1/2, Full), CTO (Colour to Orange) in the same strengths, and maybe some ND (Neutral Density) gels in varying strengths (.3, .6, .9). That might all be a bit overwhelming if you get it all at once, so it might be easier if you just buy the CTB gels and have a play, then the CTO and finally the ND.
Finally, the more you read and research on things like this, the easier it is to know what to use and when. Practice is, as with all things in film making, essential, but a solid grounding in the theory makes it a hell of a lot easier.