If you're interested in learning how to play Dobro (aka resonator guitar), one of the first things you should learn is some basic Dobro tuning. In this sample Dobro lesson from the School of Dobro with Andy Hall, Andy shows you how to tune your Dobro.
From low to high notes, the standard tuning for Dobro in bluegrass music is: G, B, D, G, B, D. For anyone that plays guitar, it's the same tuning for an Open G Chord.
Here's an easy way to think about it: the 3 high notes are the same are the 3 low notes, just one octave apart. This is useful because anything you play on the top 3 strings can be transferred to the low 3 strings.
When you're tuning Dobro, start with the G and D strings. The string farthest away from you is referred to as the 1st string. This means the G notes are on the 3rd and 6th strings and the D notes are on the 1st and 4th strings. We'll tune these up first before moving onto the B notes.
Starting with the middle G on the 3rd string, adjust the note with your tuner until it matches the pitch or turns green on the display.
Then tune string the high D note on the 1st string. Once you have the high D note in tune, play it with the G string to make sure it sounds right (they should sound in harmony).
Then go to the 4th string (low D), and get that in tune. So after that you'll have 3 notes in tune and we're halfway home!
Next we'll tune the low G on the 6th string.
Watch Andy's Free Dobro Lessons
Now all the Gs and Ds are in tune, all that's left are the B notes. Here's a tip from Andy: for a less bright, less "warbly" sound, tune the B notes one or two notches flat. This will produce a more consonant, "pleasant" sound.
With enough practice you'll be able to master Dobro tuning by ear so that you won't even need a tuner, but for anyone new to learning Dobro it's a good idea to use this method of tuning the B notes last. It's easier to tune the D and G strings first and adjust the B notes against them, you'll be able to hear how the B notes should fit into the chord.