You want a 100k pot, available on ebay for about 50p-£1.5 You wire blue/positive wire to centre pin (of 3 pins) then black/ground to left pin. You can use right pin, it just changes the direction you turn the knob to dim. Its easy to solder it, just buy the cheapest soldering iron you can for a few quid. You might be able to do it just crimping the wires on the pins but not tried it. Seriously though its the easiest thing on the whole build to solder. Part 7 of 7 of growmau5 DIY LED basics covers it well.