That's a pretty general question, but I'll try to help you out (albeit generaly)
The quickest way to modulate is a 5-1 transition. So basically, if you want to change tonalities from C to Db, the 5-1 of Db is Ab, that is, Ab is the dominant tone in the Db scale. Now Ab isn't a note in the C scale so to make the transition from C to Ab you can do pretty much anything. The dominant 7th is a good way to do this. The dominant 7th of Ab would be Eb-G-Bb-Db. You'll find the Eb in the parallel minor of C major so you can imagine how the chording would work there.
Remember there are infinite variations to modulating, but the 5-1 is a pretty good way if you're not too sure.
Try this at a keyboard though, and maybe you can work from there
C-E-G
C-Eb-G-Bb
Ab-C-Eb
Db-F-Ab
These are just the notes, you voice them however you please, but you can see the general trend. Every chord has a common note with the chord preceding it and the one after it as well. I hope this helps and if anything is unclear I'll be glad to describe in greater detail.