Removing background?

Start by reading the official help and take your time to educate yourself about render settings, setting composition background colors and a few other basics. No point in blindly following tutorials when you don't even know what they're talking about and what they leave out.

After Effects User Guide

Otherwise more specific info would be required as the others already mentioned.

Mylenium

What tutorial are you following? Most of the YouTube tutorials I help folks with are prepared by enthusiasts. The workflows can be flawed and the explanations incomplete. Please post a link to the tutorial. It will also help if you embed by copy and paste or by dragging a screenshot to the reply field on the forum. Include the modified properties of the problem layers. We need to know where the Gray background is coming from. If you can turn it off that is a start. You may need to use the Flow Chart to drill down to the background if it is in a nested comp (pre-comp). 

I hope this helps a bit. 

To start you could upload one or several full screenshots to try to see something weird in your layer settings or show us the output settings, try to give us some information that can be used to try to help you.