Security :: Building A Forum From Scratch - Registering Users To Database

I am in the process of creating a forum design from scratch, as my first official asp.net application attempt.

Overview:

I would like to handle this task in the following steps:

1) Get input from the user: txtLoginEmail, txtPassword

Form has been submitted:

2) Process inputs a) cut/trim white spaces

b) encrypt password by using SHA1 hashing method located in clsLibrary

3) Check the email address given against the database

Match: Display a message saying that the username already exists.

Doesn't: Add the email address and add encrypted password into the database

Question:

In my Register.aspx.vb code-behind file, inside of the question mark lines, how exactly do I check against
the database to see if that user exists in that database already? I thought perhaps storing the results in
an array and then looping through it to look for matches, but that sounds inefficient. I tried this idea
and after an hour I was unsuccessful. Below is my code for the class objects and pieces of code relevant to this task:

Database Design
---------------
UserID int IDENTITY(1,1) PRIMARY KEY,
UserEmail varchar(50),
UserPass varchar(50)
[code].....