My suggestion would be to contact
http://www.safari.cc/tato and review the list of members of the Tanzania Association of Tour Operators. Many of the members listed arrange climbs of Mount Kilimanjaro.
There are also some factors to consider... - Number of climbers in your group - How much time on the mountain - What is your budget
A simple Marangu route climb for one person on a tight budget might be best arranged at one of the hotels near the Maragu Gate. If you have five people that want an off the beaten track route with full camping services, then you need to consider a company that specialized in this.
Best regards and have a nice climb,
Gary
http://www.safari.cc For all the information you need on East Africa