Hopefully,you have valves under the fixture that you can turn off the water supply. If it is a single handle faucet, it probably has a cartridge inside it with bad "O" rings. If it has two separate handles, it may have washers on the bottom of the stem that have deteriorated and need replaced. Let us know which type you have for better info to fix it.
If you have never made a mistake, you haven't done much.
I noticed your pic on plumbingzone. just thought that you may not know that the two flat head screws on either side of your valve are actually integral valves and alot are just quarter turn so the slot should be horizontal. I saw that the left side "hot" was vertical. give that a try, may help ya.