Here are the rules for the formation of the Imperative form of verbs:
  Add -ќ (stressed) to the non-past stem if its stress pattern is E or M.
  Add -й to vowels and -ь to consonants if its stress pattern is S.
Infinitive Stress 3rd Plur. 1st Sg. Imperative  
  (Non-past)        
позвонќть E позвонїт позвонє позвонќ ring, phone
спросќть M спрћсят спрошџ спросќ ask, inquire
открІть S открћют открћю открћй open
постђвить S постђвят постђвлю постђвь put, stand
For the вы-form add -те (позвонќте, спросќте, открћйте, постђвьте).
The rules for making Imperative forms are stated in more detail in Imperative forms in detail.
Here is a list of verbs with irregular imperatives:
Infinitive Irreg. Imper.  
љхать поезжђй go
дђть дђй give
љсть љшь eat
давђть давђй give
пќть пљй drink
стоїть стћй stand