What you can do to begin learning the notes on the second string is to find the appropriate note on the fifth string, and count over three strings, and down two frets (see image above).
Let's use C as an example for finding the note name on the second string. Knowing that C is on the fifth string, third fret, we can count over three strings, and down two frets to see that C is also on the second string, first fret.
While this is a perfectly legitimate way to start learning the note names on the second string, I find this to be slightly tedious. You should instead opt to just memorize the names of notes on the second string, the same way you've memorized note names on the sixth and fifth strings when you began to play guitar.
Back to B.B.
Find the root note now (let's pretend we're in the key of A - so find A on the second string). Fret the note with your first finger, and play it. Now, play it again. And again... and again. Get used to it - B.B. likes to keep it simple, and you'll hear him coming back to this root note constantly.
Perhaps the most important thing to take away from the basic B.B. King hand position is King's interest in playing the root. Most of his blues riffs end on the root, and yours should too... it gives the riff a feeling of resolution, and feels "final".
In addition to the learning the notes on the second string, you'll want to learn where the root is, one octave up, on the first string. B.B. likes to slide up to this note at the climax of his solos.
