Help me figure out what's wrong with 2000 Cadillac please

There's multiple possibilities of what the problem is and since it attempts to crank from the relay, it's an intermittent failure. It could be your ignition switch, your key, the security system, crank sensor, or bad wiring connection somewhere.

When battery light comes on it is telling you there is something wrong with the charging system nine times out of ten it is the alternator and since the check engine light is on have the codes read places like auto zone will check for free.