Replying to Canon Lenses?

Just want to preface this by saying I just got into film, so I am definitely not experienced in any way. Any help would be appreciated!

So I've been filming with a t3i, and whenever I film and the light is not absolutely perfect, I find that the image quality is pretty weak. I am shooting with a stock lens, so I guess I'm wondering if thats the reason? If so, do you guys have any recommendations for lenses that will improve the sharpness/clarity?