problems with selections

I've been having trouble making decent selections with the free lasso tool. 
I've specifically been using the autopaint feature, and trying to select 
portions of the image to allow specific autopaint effects. Sometimes the 
selection comes out as i.e. a rectangle or square instead of following the 
path of my selection. Other times I'll get a strange horizontal strip up 
above the path of the selection that I've made. Probably there have been 
other permutations, but those are the ones that stick out for me. Thoughts? 
Thanks.