Do a google search for mbrowse and install it. It's a gui that will help you get to where you want to be.
On the left, mbrowse will look kinda like a folder tree. Highlight a folder on the left and on the right it will show you what that oid means. Use this tool to walk the tree from any spot in the tree. When you find the oid that will give you the desired information, then that is what you will use in the check_snmp.
It's important that you specify the MIB to use, in the check_snmp command, otherwise, check_snmp will look through all of the MIB's that are installed on the linux box. If you don't have the MIB needed then you will have to download it and install it on the linux box.
This will not be easy for a first time user for snmp, so be patient, read, learn, and in a few weeks, let us know if you have ran into a wall, that you just can't figure out.