Ok, so first thing is to read over the wiki, start with the first part on writing your first SourceMod plugin and then re-read the wikis on ConVars and menu handling. Second thing is to search through the API and look at any functions you have questions over. Third thing is to find some good examples to see how other people did things. The best way to do that is search through some of the approved plugins. Here's an example of using the ConVar in a simple plugin, look through his SP file to get more ideas.
http://forums.alliedmods.net/showthread.php?p=1097121