--Get the inexpensive bubble balance all hardware stores sell, it will let you know when your dropweight bar is horizontal.
--Acquire a tension tester--a Stringmeter, to start with, probably; tension consistency is everything, whether it's for you or someone you eventually string for--so you can establsh how you did. Learn how to use one, though.
Other tips, for consistency and accuracy:
--Opt for two-piece stringing now, when you are starting out.
--Tension your string a good 4 lbs. tighter (to allow for your floating clamps).
--Be sure you alternate mains and crosses, then do one each time.
--There is a difference between pulled, or reference, tension and what the actual tension in the strung racquet is going to turn out to be; learn about what it means now and save yourself some trouble later on...
Here is Stringmeter's version of what that comes down to, for example:
http://stringmeter.uniquesports.us/dynamics.html
--Here's another inexpensive tool you'll likely want later on, too (in some form):
http://www.eagnas.com/maxgen1/etest.html