Beg/borrow/steal an OBD scanner and see what error code(s) (DTC) show up. If you don't want to buy one, most auto parts stores will loan/rent you one for next to nothing.
Write down the DTC(s) and then clear using the scanner.
Edit: Once you have the DTC(s), head for Google.
Once you know what the car thinks is wrong, you can start troubleshooting.