A big obstacle when your campaign begins is to give to characters the right motivation to band and go adventuring together.
There you can find some good reasons you can give to your PCs.
PCs are long time friends and they know and trust each other.
They come from the same village or they've grown up together or their families have strong ties between themselves or at last they're relatives.
Army or special unit
PCS are in the same military unit (guards, scouts, regular army...) or they belong to the same special group (spies, diplomatic corp...).
They must obey to the same orders.
It's quite the same of the previous one, but there PCs obey to a religious organization, such as a church or a cult.
PCs are on someone's payroll, at least temporarily.
They were hired by a politcal organization or by some private like a noble or a merchant or a wizard for a very specific job.
The village or city or realm of the PCs is under siege or threat.
They must cooperate to defend their beloved country.
PCs are in a very dangerous situation and they can stay alive only by staying together.
They can be runaway prisoners or stranded on a mysterious land or be in a foreign country.
PCs have receveid messages by dream or by other means from their god, their patron, one of their ancients or other entities.
They're charged to join other PCs to complete a quest or to fulfill their destiny.
Ok, I know, it's an old cliché, but for some adventurers there's nothing better than a fight in a tavern to start a good friendship or a long wonderful adventure.