Just guessing here, and I hope the OP will forgive me if I'm incorrect in my assumption, but I'm thinking he might need a more straightforward answer than those given.
Think about the scale of C major (no # or b). You know, doh - re - mi etc.
1 - 2 - 3 - 4 - 5 - 6 - 7 - 8
C - D - E - F - G - A - B - C
The
dominant is the name given to the 5th note of the scale (G in this case). Remember this bit, it comes in again later.
B is the "maj7" (seventh note of the major scale). Or, to put this more correctly, the interval between C and B in this scale is a major seventh interval, but don't worry too much about this terminology for now.
Also, it's worth knowing that the note Bb, not included in this particular scale, can be called the minor seventh (strictly speaking, this is the interval or gap between C and Bb). You probably already know that the minor versions of this scale would include Eb, so this note is the minor 3rd (or, to put it another way, the interval between C and Eb is a minor 3rd).
So, some of the related chords are named as follows.
Cmaj7 = C-E-G-B
Cmin7 = C-Eb-G-Bb (notice that the 3rd and the 7th are both minor)
C7 = C - E - G - Bb (major 3rd, but minor 7th; I put this one last because although it's a very common chord, learners often find the name confusing as it gives less clues about what notes are actually in it).
The "dominant 7" chord means the last of these sort of chords, but moved up so that it has the
dominant as its root, which would be G7 in this example, G - B - D - F. Notice that in C, the dominant 7 chord of G7 can be called "diatonic" because it only includes notes from the C major scale. Not important right now, but once you know how to spell chords (work out the notes in them) then you can start trying to understand how they relate to each other a little better.
For each type of chord, try to learn how big the intervals are between each note as this will help you play the same types of chords in different keys. For example, C - E - G - B = Cmaj7. Move every note up a tone so that the intervals between the notes are the same, and you get D - F# - A - C# = Dmaj7.
Also, try to find some chord charts for songs you like and work through them, using this to give you ideas for notes to play around with in each chord. It helps to do this really slowly so that you can experiment more extensively and hear how the notes fit with the chords at different points.
If you want some scales to try, then try notes from the major scale over maj7 chords, pentatonic minor over min7 chords and the mixolydian mode (like a major scale, but with a flat 7) over plain old seventh chords (like G7). These are just starting ideas. But do remember that good bass lines don't usually work just by dropping different scales in each bar to match the chord at that point, they have to relate to each other as the tune progresses. And you don't have to play every note in a scale, leave some out here and there. Experiment, and have fun!
Again, sorry if this is more basic than you need, but some folks might find it useful. And sorry it's a long post, but there's lot of info crammed in here. Read it slowly, and you'll get it!
