Check if username exist already [duplicate]

This question already has an answer here:

• check if row exists with mysql 3 answers

I want to show a message in side or around that the username already exists when the user goes to add the password for the account regsitration where the user chooses an already used username in the registration form itself. I searched the stackoverflow, and all had different languages being used for this. I am doing this in localhost.

This is the code:

<tr>
<td>Username</td>
<td><input type="text" name="Username" required/></td>
</tr>
<tr><td><br></td></tr>
<tr>
<td>Password</td>
<td><input type="password" name="UPass" required/></td>
</tr>
<tr><td><br></td></tr>
<tr>
<td height="29"><input type="submit" value="Register"/></td>
</tr>
<tr><td><br></td></tr>
</table>
</form>

The form name is: AddUser.php Method used is: POST The username table is: Tbl_UserLogin The column name is: Username

Please ask if any more details are required for this. Thank You in advance for any help.

Edit:

I have added a procedure to check whether the username exists already. But that is done only after clicking submit button. And also, the user have to re-enter the data from the start in the registration form.

The procedure I use in DB is:

USE [db_events]
GO
/****** Object:  StoredProcedure [dbo].[sp_userregistration]    Script Date: 06/26/2016 20:20:47 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[sp_userregistration]
@Namep              varchar(100),
@Mobp               varchar(15),
@Emailp             varchar(100),
@CNamep             varchar(100),
@Rolep              int,
@Usernamep          varchar(100),
@Passp              varchar(100)
AS
BEGIN
declare 
@msg        varchar(100),
@logidp     int,
@cnt        int
    SET NOCOUNT ON;
begin try
    begin transaction
    select @cnt=COUNT(*) from Tbl_UserLogin where Username=@Usernamep
    if(@cnt=0)
    begin
    insert into Tbl_UserLogin(RoleId,Username,Password) values(@Rolep,@usernamep,@Passp)
    select @logidp=MAX(LogId) from Tbl_UserLogin
    insert into Tbl_UserReg(LogId,Name,Mob,Email,CName) values(@logidp,@Namep,@Mobp,@Emailp,@CNamep)
    set @msg='Successufully Registered'
    end
    else
    set @msg='Username already in use'
    commit
    end try
    begin catch
    rollback
    set @msg='Error'
    END CATCH
    select @msg as message
END

Please do help by showing where to make do the change, as I am a noob in programming.

Thank You