Stretching in the vegetative stage and for seedlings is often caused by having your lights too far away from your plants, and/or from having too little light. In the flowering stage, though, an initial stretch is a good thing and is expected.
There is a good bit of information on stretching (good and bad) here:
http://www.420magazine.com/forums/how-grow-marijuana/71170-what-stretch-how-can-i-minimize.html
You really haven't given us enough information, though, to be able to give you any specific advice. If you want more help, don't hesitate to post again with details (and photos are always helpful!). The age of the plant(s), the light you are using, whether you are using soil or hydro, etc. would all be helpful to know.
As a new grower, you'll definitely want to spend a TON of time reading and learning on this and other forums and websites, particularly before you put too much time and money into a grow. Consider starting a grow journal and/or following someone else's journal who is at the same grow stage as you.
Good luck!