Q: How do you tune it?
A: The lap steel comes from the factory setup to tune to Open G (D-G-D-G-B-D). Blues and Rock players tend to favor Open G and Open A. Open A raises each of the notes a whole-step (2 frets) to E-A-E-A-C#-E.
Open D and Open E are also commonly used.
Open D is tuned D-A-D-F#-A-D, and open E is a whole-step higher: E-B-E-G#-B-E. Joe Perry of Aerosmith uses Open E on his electric lap steel. David Lindley is another player who uses transposed variations of these tunings.
Bluegrass and Country Dobro players tend to favor an altered G tuning, often called "High-G", where the 6th string is tuned up to "G" instead of down to "D", and the 5th string is also tuned up, to B: G-B-D-G-B-D. They also sometimes raise it up to "High-A": A-C#-E-A-C#-E.