The measurement given for the tubing will match the measurement given for the hose barb, since generally the tubing is measured by ID (inner diameter), and the fittings are measured by OD (outer diameter), although they are a bit bigger for a good fit with the appropriately sized tubing. Thus, as Graystar said, 1/2 fittings will work with your 1/2 tubing.
To attach them all you HAVE to do is push the tubing all the way over the hose barb. You don't technically need hose clamps and I have run my rig for extended periods of time without the hose clamps on, but it is a very good idea to put hose clamps on. The tubing will tend to stretch over time, thus leaving the fit looser and it more likely to leak.
Good luck setting up your watercooling!