My 2007 chevy hhr shakes when i brake & when i go over a certain speed limit.

Ok sounds like you have things going on here. Here are a few things to check but honestly if you don't work on cars normally, you need to bring this in for an inspection. Sounds like you have a few separate issues. For the shake when you hit the brakes this could be warped rotors. If you hear a rattling when you put it in gear it could be a loose heat shield or bracket. The warning lights would require a scanner to check for codes and if you are losing coolant you need to pressure test the system to find the leak. If you prefer to use a shop, then use this link to find a good shop near you. https://repairpal.com/repair-shops Alex