I am working for a used car dealer to create a database prtotype. The
information I got is:
   Different makes and models of cars are carried by the store.   
  The financing options that customers can choose from include cash,
24 to 60 month financing, or leasing.
  Customer data, such as address, vehicle prefernces and historical
purchase records are maintained on file.
  Invoices have a listing of major options included in the vehicle 

 the functions required by the database are:
  Where Is a particular make/model car available for purchase? 
  What is the average selling price for these cars?  
  What cars are availalble within a specific price range? 
  What cars are available within a specific milage range. 
  What models are most popular? 
  What type of web site did the customer visit just prior to visiting
the  online auto reseller?

  can you help me to create the tables I need and the essential
attributes for the tables?
  thanks a lot.

What you are asking seems a bit beyond the scope of this
queston/answer service. Right now your main concerns should be 1.)
what database software will you be using (msSQL, mySQL, access (god
help ou but it can be done) 2.) will there be multiple locations and;
if so, how you will transport the data between locations (manually,
real time, batch jobs...over the internet, vpn yada yad yada)  3.)
what language are you going to use a.) access the database b.)relate
all this data to the world over the web (php, asp, etc). 4.) what kind
of platform will your servers be running (LINUX/UNIX, Windows).

at that point, you can kind of get an idea of what your tables should
be like (sounds pretty easy and the data types are no problem, but if
you do not know how to design a database in the first place, the task
of setting up any kind of relational tables can be very daunting at
first... you might just need a single table for this application)

anyway, hope it helps. I am no google-paid expert, but i do this on a
daily basis :)

Sounds like quite a task...  maybe a bit much?

You should make sure your client (dealership) has appropriate
perspective and expectations.  The scope of a project like this could
be rather large (as in several third-party products already exists
that provide this functionality --- most of them run 10's of thousands
to purchase and implement)...

Anyway, in answer to the questions that make up the functional
requirements of this software package (need considerably more than a
data model)...

  Where Is a particular make/model car available for purchase?
  * You've got an relational model decision here... what are the
objects?  Looks like location (source), manufacturer, and model.

  What is the average selling price for these cars?   
  What models are most popular?  
  * These questions can be answered by designing proper queries on
historical transaction data.

  What cars are availalble within a specific price range?  
  What cars are available within a specific milage range?
  * These are queries on existing data as well, but the mileage factor
means some (at least primitive) form of spatial analysis...
alternatively, you could store distance from the dealer for location
record (much cheaper than a full blown GIS, but not very flexible...)

  What type of web site did the customer visit just prior to visiting
  * This one is an application function unrelated to the database. 
The data store is simple transaction log of visits. You can grab the
last page from most browsers using ASP (VBScript) or Java but you're
subject to user security settings

Anyway, a basic start would be like:

Table: Source (name, address fields, type, distance, PK)
Table: Manufacturer (name, notes, PK)
Table: Model (name, year, notes, PK)
Table: Features (name, PK, FK)
Table: PurchaseTransaction (type, value, ModelID, date, etc.) 
Table: Customer (name, address, etc..)
Relationships (build R-tables M-M relationships):
  Source->Manufacturer, M-M
  Manufacturer->Model, 1-M
  Model->Source, M-M
  Model->Features, 1-M
  
and so on and so forth... that's about 0.05% of what you'd need...
hope you're not in too deep...

Try stepping back a little... the basic steps (even for prototyping)
are:
1) Set functional requirements and scope
2) Design data model (platform is unimportant... use any RDBMS)
3) Design application
4) Implement/Test

Just some insight for you... disregard if it sounds like
pontificating...

wow mike5-ga!

You sure are going all out for this used car dealer. I hope they
appreciate the sincerity of your effort.

https://answers.google.com/answers/main?cmd=search&q=mike5-ga&qtype=all

-K~

I agree with the comments from artpego and vince23 - there is a lot of
information that you still need to get from your client to really do
this project properly.  To me it seems as if you may be trying to skip
some very important steps in project lifecycle management.  Proper
concept documentation, requirements development, and project control
among some of the steps that don't seem to be finished yet.  It's
vitally important to complete these steps before moving on to the
construction phase.  I would recommend getting your hands on some good
project lifecycle books, take a look at Software Project Survivial
Guide from Steve McConnell (www.construx.com) or any number of texts
that feature well known lifecycle methodologies such as the classic
waterfall, Boehm spirals, or IEEE (a little daunting).  If you don't
have these requirements hammered out in advance, I can almost
guarantee you'll be doing this project over again, and neither you nor
your customer will be pleased.

ps - before beginning any type of work be 100% certain you have a
signed contract!