SQL Server database forum design sample
You might have something like this. If the post is a new topic, ParentID and ReplyToID would be NULL. For forums it is often desirable to show when a post is a reply to another post, but still indicate which topic it belongs to, so a second column (ReplyToID) can indicate this. When it is a reply to the original parent this can be NULL or have the same value as ParentID.

CREATE TABLE dbo.Posts
(
    PostID      INT IDENTITY(1,1) PRIMARY KEY,
    UserID      INT NOT NULL FOREIGN KEY REFERENCES dbo.Users(UserID),
    ParentID    INT NULL REFERENCES dbo.Posts(PostID),
    ReplyToID   INT NULL REFERENCES dbo.Posts(PostID),
    Content     NVARCHAR(MAX), -- more accurate than "description"

    -- ... other columns ...

    -- "DateTime" is not a good choice for a column name
    create_date DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
    modify_date DATETIME NULL -- or same default as create_date
);