Lots of ways, spoke calculator, ask the wheelbuilder at the shop, go to the mechanics forum, tell them what hub and rim you have, they canfigure out the two lengths for you (2 lengths req'd for a rear wheel build, drive side and non drive side depending on rim).

The thickness is up to you, some like straight gage, some like them butted, some want black spokes, some ss finish. Brand, DTSwiss, Sapim or other brands.

Some of the posters here are good wheel dudes. Urbanknight is full of it, .................uhhhhh knowledge I mean!...He'd tell you more about spokes.