Not everybody is on here all the time. So please be patient until someone is able to provide you with an answer to your question. Sometimes that can take anywhere from a few minutes to a few days.
Also, use more descriptive titles when creating posts that help describe the problem you are having. Similar to the new title I gave your topic.
Now to answer your question.
How are you currently retrieving data from the database: ie. Data Adapter and data set, hard coded SQL statements, etc. ?
Are you familiar with writing SQL statements?
As far as checking the database for usernames and passwords, you will need to write some SQL statements using the WHERE clause and providing the information that you are looking for in the database. If you don't know how to write SQL, then you will need to set up data adapters to retrieve the specific information that you want from the database.
You might try reading some of the following tutorials to get you started with ADO.NET.
http://www.developerfusion.co.uk/show/4278/http://www.developerfusion.co.uk/show/4286/1/http://www.developerfusion.co.uk/show/48/It will also be beneficial if you supply the code that you have currently created in your attempt to complete your project. Then we can see where you stand and offer suggestions as to what you need to do.