A data warehouse is a database geared towards the business intelligence
requirements of an organisation. The data warehouse integrates data from the
various operational systems and is typically loaded from these systems at
regular intervals. Data warehouses contain historical information that
enables analysis of business performance over time.
http://www.google.ca/search?hl=en&lr=&oi=defmore&defl=en&q=define:Data+Warehouse

Generally, to create a data warehouse, you have to focus on your user's
needs. Create the appropriate data warehouse schema, then fill it.
Specially if the operationnals systems are complex and not accessible.
for the question "designing a data warehouse from an operationnal database"
is not possible because you have to transform your data to answer the user
needs.

"converting" a database schema to another is not possible without manual
work. How the system can know that 3 tables will be merged into 1
destination if a developper don't map the columns?

if your source is simple (1 table with the products, 1 table with the
customer list etc...)  and required no transformation, you may want to
provide a real time data warehouse solution to give access to you users.
In this case, you can setup analysis services only and connect it to your
operationnal database. Analysis Services provides a wizard to create the
dimensions and cubes for you.
This is NOT a data warehouse solution, you'll just provide an access to the
information through OLAP Cubes.

Hi Maryam,
I have published a lot of materials and links for newbies at
www.peternolan.com.

If you are so new you don't know how to create a database based on
another database my recommendation would be to get some help.  You are
starting a long way back on the learning curve.

Best Regards
Peter