Help! The cache directory is currently not writable!

Im new to this so any help is appreciated. Thanks in advance. These are the errors I get once I upload everything and go to the setup page.

The cache directory is currently not writable! In order for PunBB to function properly, the directory named cache must be writable by PHP. Use chmod to set the appropriate directory permissions. If in doubt, chmod to 0777.

The avatar directory is currently not writable! If you want users to be able to upload their own avatar images you must see to it that the directory named img/avatars is writable by PHP. You can later choose to save avatar images in a different directory (see Admin/Options). Use chmod to set the appropriate directory permissions. If in doubt, chmod to 0777.

How do make the cache writable? what is chmod and how do I use it?

Thanks again