Over at Stratics, there's a good guide for learning how to craft, modify and disassemble items in Tabula Rasa.
Crafting was relatively broken in the Tabula Rasa beta, so I'm looking forward to creating enough paint to drive Dulux out of business. Delicious, delicious paint.
Several TR veterans have also recommended you start disassembling weapons for their components immediately -- whilst it loses you some credits in the short term, it'll give you a wider range of modification ingredients when you're a higher level and with a Supersonic Rocket Launcher Flamethrower of Doom that you want to upgrade. Try it out.
Be sure to read the end of the guide too, if you've been curious what the numbers next to a weapon attribute mean!
EDIT: fixed link, tricksy blogsmith