I lost the key to my 1977 Yamaha DT 125 Enduro that was the only key I had and I don't know what to do 

Did you take a note of the key number?

If you did you can order one through Yamaha with the log book / title. It should  be on the ig barrel too but 30 years may have made it illegible. May be best to buy a new switch there are plenty online.

on these old bikes you can read the key number at the face of the lock...yeah really

 

so get the number and  look on ebay?

 

http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p3984.m570.l1313.TR11.TRC1.A0.H0.Xyamaha+keys.TRS0&_nkw=yamaha+keys&_sacat=0

 

lf you take the lock into a half decent locksmith they can cut a key by probing the barrel. My bloke does it for about 15quid and it takes about 10 mins.

They have 2 ways of doing it, they have on-line records with all the #s (last one he did for me was a Z900) they take the number and cut it that way. He did that and it didnt work, so he took the lock apart and found some herbert had swapped the plates around. You have to understand these locks are about as simple as it gets for these guys. Also back then there was a limited amount of keys made, i think kawa had like 20 per year or something ridiculous like that.

lf you dont have a good smith, just find a unit on evilbay