A little tutorial i made for making a source mod!
YOU WILL NEED:
Source SDK
Hammer (comes with source sdk)
Some Hammer knowledge
A game that uses source (i recommend Half-Life 2: Episode 2)
Notepad (Notepad++ will make it easier)

INSTRUCTIONS:

~Go in source sdk and choose the 2006 or 2007 (2007 is best, and if you have orange box use it)

~Hit make a mod, and make it in C:/insertmodnamehere

~Choose your mod name

~wait for it to set up

~Go to the mod files (in sourcemods) and go to gameinfo.txt

~Change it to the name of your mod (note that it might glitch ingame, look up something on that)

~Using hammer, make a map or something

~Put it in a "maps" folder in the game files

~Go in CFGs and choose chapter1.cfg (notepad might work, i recommend notepad++ though)

~delete whatever is there and type "map (your map name)" then save

Thats it.
If you get the right stuff for real coding, great, but this is just my easy method.

If you would like to replace some models from HL2, just get the replacements, then the directory name of the original model, and where you found "maps" make a folder called "models" and "materials" then place the stuff in there and change the filenames to the names of the old models. If you do this right, the new models will override the existing ones.