It all depends on what kind of call you want to drive (single or double reed) the reason I say this is because you do blow them differently. Here is what helped me and what I haved used and continue to use.
Kieth Allen's - this is a great instructional tool if you want to be using a double reed. You can use it for a single reed but it will not give you the full potential a single reed can achieve!
Barnie Calef calling ducks my way - IMO this is the greatest instructional video to use if you want to get the most out of a single reed duck call. This instructional goes into depth on how to blow both single and double reeds.
Hobo- This is a great instructional but is not for the first timer. Cullum does a great job don't get me wrong but it isn't for the guy who is just starting out.
Remember these are just my opinions and NEVER try to sound like the person you are trying to learn from it will just make you frustrated. The reason I say this is because you could be using a different call, it is tuned different or you may just not blow the call the same way. If it sounds go to you go with it or ask some one good at calling and ask them how you sound.
If that doesn't light your fire, your woods wet!