Good ICO-company will offer you different packages. It can be only marketing, can be support in official channels, they can prepare all documentation for you.
Talking about marketing part you generally need: buzz around your project and large community of supporters.

My top-3 of marketing agencies that work with ICO is:

AmaZix is the most popular one, the biggest team, lots of ICOs
FoxTail - very reliable contractor, fair price
ModernToken - best advisers on the market, very strong marketing team.