Assuming you know how to get to the contacts list, go there first. Then while in the contacts list, press the Menu button. This is a physical button out of the 4 that are on the front of the phone, and it is on the far left (all four in order: Menu, Home, Back, Search). A white grid of choices should come up at the bottom of the screen, and one of them should say "New Contact". The icon for that choice should be a black circle with a "+" in the middle of it. Tap that choice, and the rest should be self explanatory.
Keep in mind when doing this, if you've already tied a google account to the phone, or if you've tied multiple google accounts to the phone, you can tap the top of the new screen where it says "Contact type" to choose which account the new contact will be synced to. This is advantageous to you, because if you stay consistent with which account you choose, you can add a contact either online through Gmail, or on your phone specifying that account, and when you have background data and auto sync enabled, it will automatically add that contact to the opposite location (if you add it to your phone, it sends it to your google account, and vice versa). This makes it so if you ever lose your contacts on your phone, you can just sync with your google account and you'll get them all right back.