Before getting a dog, you should...

What would you recommend to someone to learn before getting a dog?
What are the things he/she should have or let go of before becoming a dog partner?

I recommend reading all of Patricia McConnell's books as a foundation. Learn about clicker training and positive reinforcement.

What one should have depends a lot on the dog, its "job" and the interaction. Every pet owner should have:

Patience, consistency, common sense, plenty of time and energy to devote to said dog, the proper space, proper means to fund the dog, stability, etc.

Really basic things.

Let go of? The conception that dogs are easy and don't take a lot of effort. Dogs are extremely time and energy intensive.

They just need to know that a dog is like a child. For me it is as simple as this -- either you have that love or you don't. If you do, you'lre going to do what's right and all will be well.

Before getting a dog you should understand that this is a responsibility for the lifetime of the dog (say 15 years). You should be prepared for the financial commitment, food, routine veterinary costs, unplanned vet bills, medication, insurance, grooming/clipping (depending on the breed) and costs for kennels or pet care each time you take a holiday (or if you are ill). Then you need to be sure that you can devote sufficient time, effort and attention to the dog, including correct feeding, regular exercise, training etc. Finally you need to think ahead and anticipate the changes in your lifestyle over the dog's lifetime eg starting a family or relocation, then choose a breed of dog that will fit in with your lifestyle and be able to adapt to the changes if and when they happen.

Very good points on this thread, mine would be research your breed of dog throughly and think about how it would fit into lifestyle and your home - if you are an active person you want an active dog, if you want a dog to just potter round the park with for half an hour then don't look at a collie for instance. If you want a dog that is happy to walk by your side, then maybe not a beagle. If you place high value on your furniture and clothes don't look at breeds that are known to slobber - boxers! Speak to owners of the breed you are interested in or with a trainer who runs a dog training classes for their view on the breed of dog you want. Research first, dog second.

As for letting go, I don't know if you have had dogs before, but if you have, then you have to let accept that the new dog will be different from your old dog, even if the breed is the same. This is sometimes quite hard for some people to understand. Nothing will ever replace your past dogs, and whilst we all hold onto the precious memories we have of them, you have to let go of any ideals and accept that this is a new dog who needs to learn how to live with you and what is expected from you. Hope you understand what I am trying to say here, I don't think I explained it very well.