Sounds like you just want to select a value in a dropdown control (or something similar) then populate some details based on the value that was selected? There are a few ways you can do this, but perhaps an easy way is using SharePoint list data connections. If your lookup is in a SharePoint list you can use queryFields to do a search on the list based on some criteria. Run the query and the list items that match the search will be returned. Then you can use this data on your form. There are more details and tutorial out on the net on how do set this up exactly.
Otherwise, perhaps you can looking at web services or other type of data connections to help you solve your problem. Good luck!