My layer has a transparent background and has irregular borders and I want to use a brush that stays within the borders of the layer (So when I "color in" parts of the layer, the pre-existing shape is still intact but has color on the inside). I'm still learning how to use everything so any help would be very much appreciated!
If I understand you correct, you want to use "Lock Transparent Pixels". First click on the layer you want to colour. Then look above all the layers and you will see "Lock:" with a little checkered icon. Click the icon. Now when you use your brush it will only paint on the existing pixels, not the transparent area.