Not to echo what everyone else is saying, but a umbrella + light stand kit is a great place to start. Since you have radio triggers, no need for a TTL cable (unless you are in situations that you have to have TTL), manual works best most of the time anyway since it's repeatable - you just have to walk over to your flash to change settings, but that's normally not a big deal.
Along with the light stand + umbrella kit, I'd recommend getting some cheap gels (some color gels for effects, and CTO/CTB gels for colour correction). After that add a reflector before another flash, and learn how to use that. I've found most of what can be done with two flashes can be done with 1 flash + $30-40 reflector. 5-in-1 reflectors work best, since you can use them as a flag as well - I own a few.
If you plan to work outside with your strobes for portraits, consider investing in some ND filters, or a TTL radio trigger/TTL cable that will support highspeed sync - then you can do nice shallow depth of field portraits. Personally I use ND filters to keep my shutter speed down, and then buy cheap manual flashes.
The resources listed already are great, and most of all experiment! Experience is the best teacher when it comes to mastering strobism and lighting in general.
