The easiest thing it to tackle one thing at a time.
If no one has messed with the valves, then they may be out but I doubt that this is the problem.
1). Check you have got a spark - Take out the plug, hold it against a good earth (the engine head?) and see if it sparks when you kick the bike over. be careful and hole the plug with an insulated pair of pliers. Take not of the spark colour. Make sure the plug is clean and is gapped correctly.
2) if you get a spark that is blue and "fat", pour a little petrol into the plug hole, replace the plug and HT cable and kick the bike over again. What happens?
3) After you have tried to start the bike, if it is still not starting, take out the plug, clean and dry it. Return the plug into the hole and try and start the bike again. After a few kicks remove the plug and see what it looks like. Is it wet? smell it, does it smell of fresh petrol?
If you don't get a spark when you do number 1 then the issue is electrical and so you need to start checking that. A new plug and plug cap never hurt.
Do this and report the findings back.
As for the electric start, clean all of the connections from the battery, past the relay to the starter before you start to look to replace this. You can use a thick cable from the +ve of the battery directly to the positive side of the starter motor to test if the starter motor spins. Be careful here, a thin cable will get very hot and may hurt.
You must have some degree of mechanical and electrical knowledge to do these tests safely. You need to decide if you can carry these out safely.