I need some advice in selecting a statistical test please.

I am researching ambulance utilisation rates at a population level (defined by a geography)

For each geographical area i have the dependent variable: number of emergency calls made (i have the population size so i can standardise the geographic area giving a calls per 1,000 of population figure)

I am interested in understanding how a number of independent variables taken from the census for the geographical area relate to ambulance call rate figure:

Independent variables, which are all categorical are:

Age

Gender

Ethnicity

General Health

Deprivation

the variables are likley to interact with each other ?confounding

I am interested in understanding:

1. interaction of the variables to explain utilisation rate

2. where populations are similar i.e. independent variables are the same what amount of variation remains

Any advice on which methods to look into would be helpful.

Many Thanks

John.