Forgive me for my questions but i am still an extreme beginner with VB

I'm just practicing some programming so i decided to code a simple program which will help users convert between different units of measurement.
I started with Temperature conversion and what do u know i got stuck :(.
I am using combo boxes (which the user will use to choose his conversion type) and according to what he chooses the program will perform the neccessary computations. To cut all this explanation i am going to attach the program..

Thanks for all the help.