But what scales should I practice
all of them!
Here's what I do everyday, this is basically my daily warm up(after prayer), my teacher turned me onto this.
1 key a month, start with the key of C, and progress around the circle of fifths.
2 octave scales, work the scales starting on each note of the scale: C-C, up and down, D-D up and down all the way to B. Stay in the key of C, you are practicing the modes.
Both hands, but sometimes it's good to separate the hands for the 1st week to get the left hand going since it is usually the weaker.
Use what the other poster said about the metronome, start slow and build up. Before you increase the speed, make sure your scales are smooth and legato, on piano, increase the volume so that the top note is the loudest note, the lowest note the quietest.
On organ, scales are a good way to practice playing smoothly and not fumbling notes, which is easy to do coming from piano. Practice your scales on the pedals. I have a pedalling chart if you need the heel toe pattern for the chromatic scale.
Very Important: There are several ways to finger a scale. Pick a fingering and stick with it. You can't play scales fast or fluidly if you are trying to decide what fingers to use on the fly. Practice the minors!
Relative minor to C major is A minor.
All the forms of the minors, melodic and harmonic minor scales are found at:
http://www.teoria.com/reference/scales/03.htm 3 other scales I practice are chromatic, whole tone scales and diminished scales.
1 Chromatic, start on the note on which ever key you're in for the month.
On the whole tone and diminished, practice those off of the 5th degree of the scale, in C that would be G:
2 whole tone: G-A-B-C#-D#-F-G
note that there are no 1/2 steps, only whole steps between each note.
3 diminished scale G-Ab-Bb-Bnat-C#-D-E-F-G
note that the pattern is 1/2-whole-1/2-whole etc
Introduce these to yourself after the 1st year of practice.
some of you may know other versions of the diminished scale. It seems like for every book I've read that teaches it there is a different version. I know Jamey Aebersol teaches another version similar to this, I like this one because it's easy to remember, I got it out of Joey DeFrancesco's Concepts for Improvisation.