I'm with Tomas, I can barely sling one handed. I have tried left handed with really poor results. As far as not hitting, slinging has a very steep initial learning curve, I actually could throw further by hand than with a sling when I first started. Jaegoor helped me a great deal with advice and encouragement. The other members were also very helpful.
So, as far as advice goes, how are you throwing? If you try to throw at about 1/2 to 3/4 of your normal strength, you should see an improvement in your accuracy. Normally if I start to hit left and right, it's because I am speeding up my sling at the last moment, trying to get a little extra pop or snap in my movement.
As far as weight of the projectile, there is no set weight, what ever feels comfortable. I try to keep mine over about 25g and under about 100, if you want to be accurate, use all the same weight, within about 15g or so. Tennis balls are great for several reasons, they are a good weight, they are easy to find, they don't go to far, they are fairly cheap, and they won't kill somebody if you hit them by accident, nor will they break your windshield if you hit it by accident. Ask me how I know that last one.