hi there.
i write a project using VS 2008 and Sql 2005.now i want to deploy it and make its setup.what should i do for sql part?it's too big to install this mass(Sql) on user system.i heard i have to use sql engine? is this true? if yes,what's that? and how i can use it?

You end users\stakeholders\clients must have SQLExpress formerly called MSDE (Microsoft SQL Server Desktop Engine). So you don't need to install SQL Server (Server tools).
Then how to attach the database there are a lot of ways to do it programtically may be through installation, may be standalone application do that work.
http://www.codeproject.com/KB/database/DBInstaller1.aspx

What are your disk space requirements on the clients PC? I deploy a number of applications with SQLExpress as Ramy mentioned and have not had an issue with users running it.

If your download SQL 2005 Express here are some setup parameters to get you started:

setup.exe /qb ADDLOCAL=SQL_Engine,SQL_Data_Files,SQL_Replication,Client_Components,Connectivity,SQL_SSMSEE INSTANCENAME=<name> SAVESYSDB=1 SQLBROWSERAUTOSTART=1 SQLAUTOSTART=1 AGTAUTOSTART=1 SECURITYMODE=SQL SAPWD=<pass> DISABLENETWORKPROTOCOLS=0 ERRORREPORTING=1 SQMREPORTING=0 ADDUSERASADMIN=1 INSTALLSQLDIR="%ProgramFiles%\Microsoft SQL Server\"

You could likely leave out
"SQL_Replication" - Replication services
"SQL_SSMSEE" - SQL Server Management Studio Express

Building the installation parameters for the database at runtime:

Result := '"' + Trim(wwDBEditSetup.Text) + '" /qb';
  Result := Result + ' ADDLOCAL=SQL_Engine,SQL_Data_Files,SQL_Replication,Client_Components,Connectivity,SQL_SSMSEE';
  Result := Result + ' INSTANCENAME=' + Trim(wwDBEditInstanceName.Text);
  Result := Result + ' SAVESYSDB=1';
  Result := Result + ' SQLBROWSERAUTOSTART=1';
  Result := Result + ' SQLAUTOSTART=1';
  Result := Result + ' AGTAUTOSTART=1';
  Result := Result + ' SECURITYMODE=SQL';
  Result := Result + ' SAPWD=' + Trim(wwDBEditPassword.Text);
  Result := Result + ' DISABLENETWORKPROTOCOLS=0';
  Result := Result + ' ERRORREPORTING=0';
  Result := Result + ' SQMREPORTING=0';
  Result := Result + ' ADDUSERASADMIN=1';
  Result := Result + ' INSTALLSQLDIR="' + Trim(IncludeTrailingPathDelimiter(wwDBEditTargetDir.Text)) + '"';