Garry's Mod Crashes

So,my friends have been begging me to buy Gmod because it’s so awesome. So,I decided to dip into my savings and get Half-Life 2 Source and Gmod. I press play on my steam client. The screen goes black,and the loading screen pops up. I wait about 5 minutes,and it crashes. I thought something was wrong with just Gmod,so I tryed to play Half-Life 2. That crashed,too. Now,I have some of the best games that I’ve dreamed of right there in my face,and I can’t play them. It’s torture. Some friends say either get a new video card or a new computer,and others say clean out your hardrive. One said to re-install Gmod. What do I do? I don’t want to mess anything up. HELP

Oh,and my computer is able to run WoW(don’t judge me),Fallout New Vegas(Low Quality),and Spore(Low Quality).

What are your specs?

When you say “it crashes”, couild you be a bit more specific?
Does it flash a blackscreen?
Does it appear on the taskbar for a second?
Does it give an error message?