Select the folder you want to share, then hit the little gear/cog/whatever button and choose "Get Info." From there, hit the arrow next to "Ownership & Permissions" and press the arrow next to "Details." Then change the drop down menu beside "Others" to whatever kind of access you want the other users to have. If you're selecting a big menu like pictures or music, you can also hit the "Apply to enclosed items" button. This is how it works in 10.3.8, hopefully it's not too different for other versions of the OS.