This topic has been repeated to death. I understand you are new, and you are trying to develop your sidearm. But the BEST thing you can do is sort through the WEALTH of information that is already here in the first place;
http://www.dgcoursereview.com/forums/showthread.php?t=107828&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=106518&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=106316&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=104941&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=103980&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=80831&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=92707&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=91407&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=91047&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=90911&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=90785&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=88770&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=88770&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=61516&highlight=sidearm
http://www.dgcoursereview.com/forums/showthread.php?t=83868&highlight=sidearm
There are several ways to immediately improve your form;
#1 video yourself and watch it, you'll see flaws
#2 watch videos of pros, and compare that to your video form
#3 upload your video here for others to critique
#4 get out in the field and THROW discs, focusing on accuracy FIRST, then distance
#5 understand that OS discs are NOT the answer for sidearm throws, Scott Stokely threw a disc made back in 1998 well over 500 feet, not these ridiculous 13 speed Apes (which I personally hate)
#6 there is an unofficial record of over 550 feet with a 40 mold midnight flyer. So yeah, form means
everything
#7 Jeremy Koling repeatedly throws more "understable" discs. So again, stop thinking that sidearms are only good with OS discs.
#8 If you can't sidearm a leopard, accurately, then you can't sidearm very well (just my OWN opinion, which stinks).
#9 BEWARE of internet distance claims. Throwing a putter, sidearm, on a rope, 300 feet, repeatedly, means you should be rated well over 1000.
#10 If you want to get better, find out the weakest spots in your sidearm game, and strengthen them with learning to throw that problem area backhand, don't limit yourself.