Lessette, welcome to the forum
I'm sorry you're expiriencing such an issue, but it is quite common for us without gaming machines or without setting some things up.
First, what are your PC specifications? How much RAM do you have?
Make shore to add Sims3.exe and latest EP .exe file you have to DEP.
Here is the link to wikia explaining how to do that.Lower the settings on lowest possible and load the world - if it can play without crashing, try increasing some settings and find with what you can play (I'm talking about graphical settings in game).
Depending on how much RAM you have, you shoud dedicate more than 2Gb of it to the Sims as UC with high settings will need more than that. If you don't have that much RAM keep your settings on low.
Here is the post where Keyz explained how to do this!Turn off all uneccessary applications running in background, some people turn of internet too! antivirus, msngers of all kind etc.
Play in window mode - it is taking significantly less resources than playing in full window.
Here is the tip from Samoht too:
Just a tip for if anyone wants to try any solution they can - I solved my random crashing by turning off Verticle Sync in my NVIDIA Control Panel under 3D Settings and now it hardly crashes, seems it only does due to the internet and the game being minimized.
Hope some of these will help!