Useful pages to read through for a better understanding of Chords & Keys are:
About Chords is also very useful.
For improvising you'll need to know what key the music is in and then you can choose some scale patterns to try out.
So if we have a chord progression in the key of G major:
[G major chords are: G Am Bm C D & Em]
G major Chord Progression
G / / / G / / / Bm / / / Bm / / / C / / / C / / / D7 / / / D7 / / /
C / / / D / / / G / / / Em / / / Am / / / D7 / / / G / / / D7 / / /
you can start to improvise to the progression above using any G major pattern.
Below is one pattern for G major that starts with finger 2 at the 3rd fret
Below are the 6 main chords in the key of G major
G Am Bm C D Em
I have tabbed out below 2 more G major positions to try:
1 2
e ---3--2--0--------------------------------------------------------12--14--15--
b -------------3--1--0--------------------------------12--13--15----------------
g -----------------------2--0----------------12--14-------------------------------
d -----------------------------------------------------------------------------------
a -----------------------------------------------------------------------------------
e -----------------------------------------------------------------------------------
Bass Guitar
For Bass Guitar your scale notes are: G A B C D E F# & G !
g1 -------------------------------------------------0------------------
d2 ------------------------------0-----2-----4--------------------------
a3 ------------0----2-----3---------------------------------------------
e4 ------3--------------------------------------------------------
G A B C D E F# G
For the 3 major chords you can play the following: [Arpeggios]
The 3 notes for each chord consist of Root, Third & Fifth
g1 ---------------------------------------------------------------
d2 ----------------------------------2------5---------------4------7-----
a3 ---------2----5-----------3-----------------------5---------
e4 ---3-----------------------------------------------------------
G major C major D major
And for the 3 minor chords try these notes: [Arpeggios]
g1 ---------------------------------------------------------------
d2 ----------------2-----------------0-----4----------------------------
a3 ---0-----3-----------------2------------------------------------2---------
e4 ---------------------------------------------------0------3---------------
A minor B minor E minor
I hope this has really helped and remember if you have any questions, bring them along to the lessons.
Good Luck and Keep Practising but above all, enjoy the guitar!