Some prepositions are always followed by the accusative. These are the five you'll need most:

Preposition	Meaning	Example
durch	through	durch den Park — through the park
für	for	ein Geschenk für dich — a present for you
gegen	against; around (a time)	gegen den Wind — against the wind; gegen acht — around eight
ohne	without	Kaffee ohne Milch — coffee without milk
um	around; at (a time)	um den See — around the lake; um acht — at eight

A handy way to remember them: DOGFU — durch, ohne, gegen, für, um.

As always, only masculine words show it: für den Hund, ohne meinen Schlüssel, für ihn. (das Geschenk — present; der See — lake)