Asatru is not a place, it is the Prue-Christian faith of the northern European peoples, most commonly called heathery... which is why we're discussing it in this forum.
The Troth offers a very rigorous clergy training program, whose details are far too numerous to list here but, for those interested, the link is provided below.
Boar's Heart correctly points out that you don't have to have any official seal of approval to be godson/man. HOWEVER... if you don't want to make a complete fool of yourself in public ritual (e.g. "Oh Thor, you are so mighty with that big hammer... mum... gee, we just think you're wicked cool") then it pays to study the lore and understand what the Hel it is you are talking about.
Wassail!