One of my plants seems to be stretching more then it should be at a week and a half old. I have it under florescent grow lights but its always been a couple inches away from it. I turned a fan on them to strengthen the stem.
Try to get the light as close as possible so that It doesnt need to stretch, within an inch. The fan will definitely help, and you can bury the stem just a little with new soil.
Yeah, what Anony said . Also, some strains just like to stretch a bit for some reason. The white widows I grew last time stretched really bad at first and then they filled out nicely with no problems. Just keep the light close and keep that fan on them and you should be fine.