First things first, get a voltmeter and check the voltage of the battery. It should be around 12, preferably pushing 14.
If not, you need to try charging it or get a new one.
Once you have a charged battery, clean the terminals and the leads thoroughly with a wire terminal brush and reconnect. Try the headlights. If no headlights, check your fuses and look for corroded connections from the battery, especially the positive but also the negative lead that grounds to the frame.
If you can at least get headlights, but starter won't turn over, look at connection from battery and give the starter housing a vigorous whack with a stout stick.
If you don't get any signs of life at this point, consult a mechanic.
If you do get it started, check the oil for any water (will look a little like mayonnaise). Don't run it until you've changed the oil and filter. Then keep a careful eye on it and change again if it looks at all degraded.