If you are damaging your shoulders all the time, sonds like you could be getting your head on the wrong side. For this, practice using tackle bags (no matter what peppers07 siad), they are the perfect way to improve your technique without the risk of injury. Remember to practice tackling using both shoulders, not just your strongest side (usually right, for right handed people).
Now when you make the tackle, make sure your head is on the correct side, this is very important, or you WILL hurt yourself every time! If your tackline with your right shoulder, your head should be on the left of the player/bag. Tackling with your left, head on the right. This should come naturally most the time, but maybe not all the time.
Tackling low is a good idea, but anckles can end up with a few kicks to the head if anything goes wrong. I personally find the thighs the best area to aim for, then it's quite simple to slip down during the tackle to bring down a player of any size.
As nick_w said, don't be afraid to tackle anyone, no matter what their size. Probably the first thing my dad taught me when I was young was 'the bigger they are, the harder they fall' thing. The last thing I'll say is, always go all out into a tackle, not for a big hit, but never go in half hartedly, it's a sure way of picking up an injury.
Good luck with it!