I need some common reasons why a dirtbike wont start.?

my boy just traded his bike for a 1989 yz 180 and the person he got it from was riding with me last week with this bike. Now we tried today to get it started and it didnt even want to start. also we tried to run start it and when we popped it into gear the bike would barely move it would skid if anything . if anyone has any information on how to start this bike it would be greatly appreciated.