Learning Audio Synthesis in 5 Steps:
You can of course approach the process of learning audio synthesis in many ways. You can start on hardware or software and as long as it's a decent synth you'll be fine. Using modular synths may be one of the best ways to learn because you're working on a component level. Synth modules can get pretty pricey very quickly, so there's that fact to consider.
Another approach is to just open any good soft synth and figure it out by trial and error. This may suit your style of learning but it can also take longer since you have no structure in your process for learning synthesis.
The approach below will give you the best of both the above approaches while at the same time negating some of the cons.
Start with only one good soft synth. Then do this:
1. Use Presets
Almost every soft synth on the market comes with hundreds, if not thousands of factory presets. Go through the presets because it gives you a good idea of what your synth is capable of.
Yes, some synth puritans may tell you not to use presets in your music, and that's all well and good. You have to learn when you start out though and using presets will give you a good idea of the lay of the land.
It doesn't stop there though...
2. Analyze & Tweak Presets
Look under the hood and figure out what makes the preset synth patch sound the way it does.
Then turn some knobs, move some sliders and listen to what happens. This will give you a good idea of which parameter or component in the synth contributes to which aspect of the synth sound created by the preset.
4. Program Your Own Synth Sounds
Using presets will help you get better at audio synthesis. So will tweaking presets.
Nothing however beats programming your own patches from scratch. So, after you've spent some time with presets you'll start to get the basic flow of a synth.
The process of starting with one or two oscillators or waverforms and then processing the signal to create your own synth patches will be so much easier once you understand the basics of synthesis.
So, program your first kick patch, then a lead, then move on to a pad, a bass, a riser. Keep trying new things until you've tested the limits of the soft synth you're working with.
When you're sure you've done that...
5. Try Other Synths
The knowledge and skills you have by this time will be transferable to pretty much any synth. So, now is a good time to test different synths.
There will be a slight learning curve for you on a new synth but you'll quickly pick up the basics because the principles remain the same. Inputs, processing, outputs. Sure, some labels might be different but you'll have the frame of reference of the first synth which you've mastered by this time.
You'll quickly figure out which synths work best for you.