Great to hear you're keen to make your own boards!
In terms of software, at a beginner level I would recommend Eagle or Circuit Maker.
Particle (and much of the open source) world use Eagle. I've always preferred Altium who make Altium Designer (~$8000), Circuit Studio (~$1000) and Circuit Maker (free). But try them out and see how you go!
In terms of ease of use for Particle based designs, Eagle might be easier to start off with as I believe Particle provide libraries for their components for Eagle.
To get started I'd just recommend looking at plenty of example schematics. Adafruit, Sparkfun etc. all have great tutorials. I quite like Dave's tutorials/videos at eevblog.
I get most of my PCBs done from www.pcbway.com. However I hear seeedstudio is good too.
From what you're describing, you're kind of after a development board? If you're after something custom, it will probably be quite expensive to get partial assembly done on only a couple of boards. We usually only start out-sourcing assembly when we're making 10+ boards, even with SMT (surface mount).
You'll find that just buying a few basics, you can actually assemble SMT boards yourself, I've made plenty of boards just using a mini-oven with a brick to regulate the temperature - not pretty but it works
Great places to source components are:
www.digikey.com (my personal favourite as component search is very good)
Any more specific questions, please let me know.