if your job requires one, then your employer will pay for it.
one key thing is: TELL THE TRUTH. if you lie about anything (drugs, legal problems, financial issues, etc.) & they find out, then you won't easily get the clearance & will probably lose it if you do get it (and get fired).
even a felony conviction doesn't automatically stop you from getting a clearance if you've been rehabilitated, but lying isn't easily forgiven.