Software guy needs help with circuit

My project has an 11.1V 800maH battery.

I would like to use this to power my RedBack ( http://www.linkspritedirect.com/product_info.php?products_id=26 ) and three Maxbotix ultra sonic range finders ( MB1010 LV-MaxSonar-EZ1 | MaxBotix Inc. ).

I understand that Ohm's law is v = ir however I don't know how to apply this to my problem.

I would like to keep the power range going into the Redback between 5-7v. I don' t know minimum power requirement for maH in this type of set up. I tried using another battery for this project but it is just way too heavy so I need to build a circuit that will go from the 11.1v 800maH battery and drop it's power down. I would really appreciate some help with either numbers to calculate this or just a few links to hardware that would do the trick.

Thank you in advance,
Postholes

Google "Voltage Regulator"

http://octopart.com/partsearch#search/requestData&q=LM7805