Yah, it's pretty easy. In the US, it's handled by the RIAA, so go to their website and register for your own 'registrant code'. This alpha-numeric 3-digit code will be your unique identifier that you'll use to create an ISRC code for each song.
An ISRC broken down into 4 sections: country code, registrant code, year, then user-assignable numbers.
For example, if you are located in the US and your registrant code is XXX, an ISRC from today would look like this: USXXX1000001
US: country code
XXX: registrant code
10: year code
00001: user assignable number
As you can see you, the ISRC allows for up to 99,999 assignments per year, so it's unlikely you'll ever need more than one code per label.
I generally recommend that folks only assign ISRC's once the release sequence is completed so the numbers will be in order like the release. Likewise, instead of starting at ...00001, ...00002, etc, I recommend people start at ...00101, ...00102 for the first release. If they have a second release within the same year, they can start that one at ...000201, ...00202. While you need to keep these numbers written down for reference, if you follow this method, you can reconstruct any ISRC code assigned in your head simply by knowing which record was released when each year and by looking at it's sequence. Using this method, if you want to know the ISRC code from track 7 off the third release you had in 2002, you'd know the number is USXXX0200307.
Anyway, hope this helps a bit, if you have any other questions, ask away...