please help 2003 chevy avalanche

having trouble finding out whats wrong with my truck battery/alternater both good truck at time starts and at times wont when it does battery guage is real low truck shakes like it wants to turn off when it wont start nothing at all no noise or anything can somebody help point me in the right direction

Load test the battery, check all the cable connections, when this happens get a helper to try and start it and check the voltage drop across the battery.