How To Enter Commands
There are 3 ways to use a command. You can enter it in chat, enter it in console, or bind a key to use it. Each method has its own format, so make sure you read this!
Using Commands In Chat
First off, I should explain that there are 2 type of commands that you can type in chat. One type is called a chat trigger.
I won't go into the details, but these commands can ONLY be typed in chat. 99% of the other commands are regular sourcemod commands that can be entered in the 3 ways discussed here.
If you want to see a list of the chat triggers, see the chat triggers page.
To use a sourcemod command in chat, simply put a slash in front of it. For instance, if you want to load up the player menu from chat, you would type in /menu.
If you wanted to remove your checkpoint, you could type /ucp.
Binding Keys To Use Commands
Binding keys to do things is usually your best bet. For example, you could bind your C key to save your checkpoint when you press it.
If you do not know what a certain key is named, like the keypad up arrow, you can find out by going into your normal keyboard settings and temporarily assigning it to a standard function.
Then after you know what it is called (kp_uparrow), be sure to clear the key for that thing you just set.
To bind a key to load up the player menu, you could type something like this into your console.
bind c sm_menu
To bind a key to a command that has more than one word in it, such as to set your model to a specific model, then enclose the command in quotation marks.
So, to set your model to sonic the hedgehog by hitting your N key, you would type this in...
bind n "sm_model sonic"
To see more about binding keys and some examples, check out the binding commands page.