Other than a ball and a desire to play the game, players must have appropriate (and hopefully comfortable) footwear, shin guards, and correct-length socks.
Footwear can range from indoor soccer shoes (grooved rubber sole), to turf shoes (small rubber nubs), to cleats (10-12 molded plastic pieces on soles), to studs (6-8 metal spikes on soles). The field surface and weather conditions will determine the appropriate footwear. The flatter and harder the field, players will want flat, soft shoes (indoor or turf). Playing on grass usually requires cleats. Older teens and adults may benefit from studs, and then usually only when the game is being played on a field with exceptionally long or wet grass.
Shin guards are mandatory equipment for any official game. Shin guards can cost as little as $5 or more than $50, depending on the materials. They can range from a simple piece of plastic to state-of-the-art synthetic materials.