You can make ok samples sound great with a little tweaking. What you need to do is go visit webiste like http://www.tweakheadz.com/ and just grab some basics. Obviously you know how to input midi data, so your already ahead of alot of people.
As far as rythm stuff, whatever program your working in you need to have your song lined up with clear indications where beat one is and so forth. I always have things divided up by 1/16th notes which is about as crazy as I want to go. And if your not sure, just start everything on beat 1 to get used to it.
Keep on fighting bro. You've obviously have some talent to write music and know how to tweek stuff to a small degree, so just listen, read about it, and play around with things. Don't let anyone tell you to quit. I was tone deaf as a kid and was rejected from musicals and all sorts of stuff growing up. Now I play lead trumpet at Portland State and have played around the world. Dont take no for an answer.