Why do muscle knot form?
You have to push yoursrlf to make gains, key is to not overdue it. This is best accomplished by listening to your body. Takes a little practice.
I assume you're talking about weight training?
When I first started working out with weights it took almost a year before I could push my muscles indefinetly without suffering some soreness afterwards.
If you are just starting out do not push yourself, just have fun with it for a while till you're confident you can add more stress. And at the first sign of overstress back off on that muscle group. Remember there's always tomorrow. The gains you make will come more rapidly if you take it slow. The saying "Haste makes waste" has never applied better in this situation.
Always do some kind of warm-up before you work out. I walk a mile or so at fast pace before any long run.
With weights I do 2 light (50% of max) before I do the 3 at full intensity. Then do something in between and after your sets to keep the blood moving.
Try not to mix aerobics and weights on the same day, it's just too much, unless your just doing some light maintenance work.
Make sure you keep well hydrated and eat a well balanced diet and also very important get plenty of sleep.
Believe me I've learned that from the pro's.
Hope this helps