Can i get an std through a beverage. say if someone spits in my beverage like a milkshake before serving it to me. also i have canker sores.

Yes, and No: The virus of an std would actually be transmitted through the contact with the edge or rim of the glass. If you feel that you are getting a sore, that is when you are more contagious and you can transfer your sore to someone who shares your glass. Be safe and smart! do not share your glass with anyone! let your sore stop it's spread with you.

Possibly: If you have open sores or cuts in your mouth saliva is the carrier of many viruses . If the liquid acidic than chances are less.