I'm just getting started in this hobby and I'm shooting with a Canon XSi and stock IS lens. I'm having a hard time getting the bold colors to come out like I see in more experienced photographers photos. I've noticed I can get good color when I have a ton of light but most of the time the color is just dull. Is this because my lens doesn't let in enough light or do I need to make some adjustments within the camera?
The main things to do when shooting are (in order):
1. Expose accurately. Don't rely on making any significant exposure adjustment in post processing.
2. Make sure the lens is impeccably clean, both the front element and the rear.
3. Don't use any filter that isn't necessary; UV filters are never necessary unless there is blowing sand and/or grit.
4. Use a lens hood.
5. Use the lowest ISO possible. Raising the ISO more than 2 stops from your cameras lowest ISO should be avoided.
6. Calibrate your editor's image display so you know what your are really getting in your images.
7. Understand that you may need to adjust color saturation (if using Photoshop, Vibrance also) along with Curves in post processing to get the results you want.
Since you are getting what you say is good color when shooting in bright light, I think your primary issue is with #1 and #5. High ISOs are a problem because noise suppression often reduces color saturation as part of its method to reduce chroma noise (color noise).
You will also find that having to make excessive White Balance adjustments (e.g. balancing light that is below 2800-3000K) will result in poor, uneven color saturation and correcting in post often leaves undesirable artifacts.
Calibrating your display doesn't have to be excessively precise for non-critical work. Simply download good examples of images you think look correct in your browser and view them in your editor and in your OS's native viewer (e.g. Windows' Preview). If they look good in your browser and Preview but don't look the same in your editor you need to investigate how to adjust your editor's color display (good editors will have some control, usually by applying "profiles").