You will need a set of three boules to play petanque (although most leisure sets come as 3 or 4 pairs). A boule should fit in your hand with the thumb and forefinger just touching each other in a light grip.
Never rush into buying boules, and it is suggested that you play a while and become comfortable playing with club boules or whatever boules you have yourself. Once you have decided that petanque is your game, then look for a set of boules.
A Positioner is a player who throws the ball and tries to get it as close to the jack as possible. A good positioner in a team is quite often a deciding factor in a game, and a good positioner can counteract an attacking player simply by being consistant in placing his or her boule close to the jack. Usually a positioner will use boules that are heavier in weight (700g-720g) and with more 'striations' (cut lines in a boules surface) to grip the ground.
The Shooter is a more specialised but spectacular role (when it works). The shooter is basically a positioner who when required can attack your boule and knock it out of position, hopefully leaving his or her boule in it's place. Attacking can be a double-edged sword, if it all goes well it could decide a game at a crucial point, and can have a psychologically devastating impact to an opponent if it works correctly. However every attacking shot that misses its target is a lost boule. The shooter will usually go for a lighter boule (680g-700g) without any striations that could affect accuracy. Shooters prefer softer metal boules so that when they strike another boule the shock will be partly absorbed by the softer metal reducing rebound.
Other equipment that should be considered when purchasing boules include a boule carrying bag, a magnet on a string to pick up boules, a game score recorder, and a tape measure for measuring which boule is closest to the jack.