Forum DB Design. Advice needed on creating relationships & setting keys

Hi I am trying to create a mock forum. I've 3 tables - ForumUsers, ForumThreads & ForumPosts. I've created them as follows:

create table ForumUsers(
userID int identity(1,1),
userName varchar(50),
password varchar(50),
displayName varchar(50));

create table ForumThreads(
threadID int identity(1,1),
title varchar(250),
startDateTime datetime default getdate(),
startedByID int);

create table ForumPosts(
postID int identity(1,1),
threadID int,
postTitle varchar(250),
postText varchar ...