Most of the SFX in various big mods is my work. I've been doing this for over a decade now, so I recommend starting out how I did: by learning how to mix different sounds into a new result and going from there. With good source material you can get pretty far without having to delve into complicated things. I'd still recommend learning basic audio theory and concepts such as loudness, frequencies, envelopes, time stretching, transients and basic tools like EQ, compressors, and various filters, it's fun and massively increases your ability to do stuff.
There's no magic shortcut for learning any program, like for instance Audacity. If you have zero knowledge related to audio, it will add to the learning curve. You just need to invest time into tutorial videos, documentation and trial and error. I personally use FL Studio, which is a music sequencer first and a sound editing program second, but I find it to be extremely powerful for all purposes, especially because it has a built-in single track audio editor. Any program you're going to be using needs to be able to layer multiple tracks or samplers.
For starter royalty free source material look into the GDC Audio Bundles from Sonniss. Sadly, working with audio takes a LOT of disk space...
Some tips regarding audio in SS:
- In sounds.json, pitch values lower than 1.0 are downsampled, while greater than 1.0 are upsampled. Consider this like playing higher or lower notes on a piano, but with some differences (downsampling will make the sound longer and slower, upsampling will speed it up and shorten it, plus the frequency content is altered). It's a good idea to vary the pitch a bit for each sound. My rule of thumb is to use three pitch variations for a given weapon sound: one should be at 1.0, one at 1.03, and one at 0.97.
- Any sound with a single sample source will self-interrupt ingame, so it can be used for spammy weapons but still have a pleasant tail-end. Eg. you can list the same sample 3 times, it will self-interrupt, but if you list 3 different samples, it won't. "Sample" here refers to a sound file in the mod's file directory. You can refer to the same sample as many times as you want for each sound source.
- Any stereo sound will be played in the UI layer, and won't be placed in the game's stereo field by OpenAL, so it will sound like it comes from "everywhere". Stereo is useful for rarely-occurring, attention-requiring events, or for when you just want something to sound really badass. (I used stereo for the Scalar Deracinator shipsystem in Blackrock)
E: Also for SS sounds, format should be .ogg, 44.1khz and mono. Volume can be as loud as you want as long as it doesn't clip, as a general rule - you set ingame volume levels in sounds.json.