If Then Coding?

i made a a database and have a table with LanID's. I have a form where people will have to put in a LanID when they first click on the form. i want to make a code using If Then statements. if they put in a correct LanID then the form will open. if they put in wrong LanID. i want a message box to pop up saying something like "invalid LanID, please enter a new one." again i want to do this using IF Then Else statements. if the LanID is correct or not will be based off of the Tables LanID's

Table Name is NRCEmployees
Field Name is LanID
Form name is F_EMPLOYEE