Simple Mate or angle constraints (either using faces on the parts or their origin planes. But obviously you can't lock them into place until you know where they are supposed to go exactly. So until you know that there is really no need to constrain them. Or partially constrain them in 2 directions until you figure out the 3rd.
Have you had any formal training? I'd highly suggest it as 1 day spent in training can save you weeks of wasting time trying to figure it out yourself.