Amsterdam has 38,200 hotel beds, apparently. There are loads of hotel booking sites, so take your pick. Go for somewhere on the canals if you can and reckon on paying at least 80 euros a night for a room.

If you are looking for cheap and cheerful and you don't mind church bells:

Hotel Van Onna
Bloemgracht 102-108
Tel: 020 626 5801

Smaller, more upmarket and highly recommended

't Hotel
Leliegracht 18
020 - 4222741