Before you start making checks of the battery and electrical system, battery acid is corrosive, can burn skin and melt clothes. Also, when make electrical checks, be careful not to short any hot wires to frame. Need to verify battery voltage static and when trying to start. Static should be roughly, 12.5 volts and if the starter is cranking, about 10.5 to 11 volts, also verify all fuses are good. Make sure both battery cables are clean and tight at the battery. The voltage of the battery should read the same at the cable going to the starter. If not check connections. Always make electrical checks using the negative battery post. If for some reason the voltage suddenly drops, need to make sure all ground connections are good. Check the voltage at the solenoid starter wire. This wire should be going to a smaller terminal on the starter. Check this with the key switch in the start position. Should read battery voltage. Does the starter turn the engine over? There should be a starter relay at the fuse box. The relay is what sends battery voltage to the small wire at the starter. There may be a brake switch that is not working causing the whole problem. Need to know what the model number is to be more specific. Thanks.