Each item in the core books has a listed encumbrance value. A character can carry 5 plus his brawn score worth of items. An item emcumbrance value does not equal weight per se, but how much effort you have to make to carry it around without trouble.
And by trouble i mean one setback (black) on all checks related to agility or brawn. If you go above twice that threshold, you lose your free maneuver you get each round (unless he uses two strain per maneuver, as normal).
Certain items, like a belt with pockets, will increase that limit by one, while certain backpacks will increase it by another point or two, depending on the quality of the backpack.
Those rules are explained on all three core books, pick one of them and you are good to go. Note that the beginner box does not have rules for encumbrance.