Most clubs follow the NMLRA rules but there are always regional exceptions.
The hawk cannot have a cutting edge longer than 4", the hawk must make at least one complete turn. To keep folks from showing up with a miniature hawk and throwing from 2 feet away there will be a line that you must throw from behind, usually somewhere around 4 feet. H & B Forge makes a good quality hawk that meets the regulations
http://www.hbforge.com/products/tomahawks.php
The knife can only have one cutting edge and a straight back. The blade can come to a point, think Bowie knife here. The handle must be a natural material, wood, leather, bone. Most knifes have thick leather handles (think shoe sole here) since they are less likely to break from practicing.
http://www.hbforge.com/products/throwing-knives.php
I find that a longer knife is better than a shorter one because it throws from close to the distance as my hawk.
Some hawk matches occur as part of a woodswalk. Sometimes the block is sitting on the ground and sometimes hanging in a tree. You will need to learn how to throw and stick in all those situations. You will find that a turn and a half will cause the hawk to land handle up.
Some matches will not allow you to pace to the target for your first throw. You need to be able to learn how far to stand from a target without measuring.
When throwing in competition, the most common target is a playing card. One point for a stick in the block, 2 points for cutting the card and 3 points for cutting the card in half.
For shooting competitions the rules vary widely but these things are often part of the rules. The sights can only be steel or brass. No glow in the dark "hunters" sights. Most often the rear sight must be fixed not adjustable. Many shoots will not allow peep sights or buckhorn sights. You can paint the front sights if you need but only black or white. Range rules vary widely, so if you are shooting with a new club, it is best to ask the range officer for the rules. I found out to my surprise that in Maryland, the State Parks department thinks it is safer to have a flintlock on halfcock with a frizzen stall on it than with the pan open and the hammer down.
Have Fun
Many Klatch