Please help

Make sure battery has a full charge and battery connections all good, positive and negative. Use a digital multimeter across battery posts, everything off, the generic spec for a full charge is around 12.6 volts. Leave the meter leads touching the battery, have helper turn key to crank, how far does the voltage drop, most info says it shouldn't drop below 9 or 10 volts. If the reading drops way down there, you need to use a charger and charge up the battery. If the reading doesn't drop or not very far, then check for open circuit to starter motor. With other testing, you could jump the load side of starter relay to see if starter motor will crank the engine. If you try that make sure tranny is in park or neutral and parking brake is set. See the starter relay in diagram, unplug it, terminal 30 and 87 are the load side, Terminal 30 is hot all the time and is fuse protected. Use a test lite and check voltage on that circuit, terminal 87 to starter motor is also fuse protected, goes hot when relay is energized, use proper jumper wire between those cavities, the starter motor may crank. If I suspect a problem with starter motor, sometimes I take it off, lay it on the ground, hook up jumper cables properly from starter motor to charged battery, at the least, it should kick out the drive gear and spin. I haven't looked at your security system?