SQL Server 2014 :: Get Next Primary Key Without Identity

I have a table to store int primary keys. Where the value nextID is the value to use.I need to get the value and update it (nextID + 1) before the next person gets it.I thought using tran would work but it doesn't stop a select.How do I get a value (lock the table for the update), update the table (preventing any selects until done) and release the table.

BEGIN TRAN
UPDATE TableIds
SET NextId = NextId + 1
WHERE TableName = 'Users'

WAITFOR DELAY '00:00:20'

SELECT NextId
FROM TableIds
WHERE TableName = 'Users'

COMMIT TRAN

I really want to do this in the reverse order but it doesn't work. But if I do this and have another query to just do a select:

SELECT NextId
FROM TableIds
WHERE TableName = 'Users'

This query never returns. Doesn't the COMMIT TRAN release the lock on the table?