diffrent database, for each application

Hello, 
          I have install mail server for our company and it working well but now i am to install ii 7 for the company website which will use word press php and also we will be install another software which will also need a database software..I am not Good with database. I will like to know if i need to install diffrent database software like Mysql or miscrosoft Sql for each software .or i can i use one software and create diffrent acount for them .we are constraint with budget so we can not buy separate server.
thank
kafui