You can mix scales up, but for a beginner I think it's good to get a feel for the character of each scale, so I'd suggest picking a chord and then jamming over that chord with notes from one of the scales you know. Different scales and modes have different "flavours".
Some scales generally sound more comfortable over certain chords than over others. Over any given chord you have a number of scales whose notes may work for improvising. For example, playing over a C Major chord you could try notes from the scales of C Maj, F Maj, G Maj, C Minor pentatonic, A minor pentatonic and loads of others.
Some scales will not work too well over some chords. For example, I mostly wouldn't jam over that C Maj chord using the C Dorian mode - although I
would try that "scale" with a C minor...
Oy... this is actually much harder to explain than it is to do. Sorry if what I write seems jumbled.
It all depends on what kind of feel you want your phrases to have. Just as Major and minor chords have their own distinctive characters, the same is true of scales.
Check out the topic about pentatonic scales. Those are a great place to start understanding how scales and chords work together.
http://www.ukuleleunderground.com/forum/forumdisplay.php?f=3
It is helpful to know the chord arpeggios as well as scales. Solos sound good if they move around, and resolve to, chord tones.
Man, that all sounds really vague as I read it back to myself.
Sorry about that. Maybe someone more eloquent will be along in a while...