For free and very good results you could start Synth1. You could learn the basics in this free monster. It is subractive virtual synth which means that is pretty easy for begginers.It contains osc's, lfo's, some types of essential filters, unison and some effects. I suggest it without hesitation. It was my first virual instrument and learned a lot of things with that. Plus that there are a lot of free preset packs around the net. Hope I have helped...
I'm really in love with Synth1........