Threads

Doing threads can be a bit confusing for beginners, You have to use the helix curve feature and either a sweep or a sweep cut.

http://www.instructables.com/id/How-...three-minutes/

And heres a quick method for terminating the thread ends smoothly.
http://grabcad.com/questions/how-to-...-in-solidworks

If the thread is on a pre-manufactured component and/or ,multiple threads it may be best to use a cosmetic thread to reduce performance issues.

Dimensions

You can use a centreline to do diametric dimensions. Or you can just type your dimension followed by /2 in the dim box.
http://help.solidworks.com/2012/Engl...5d1ed7f5f712c9


I recommend buying the solidworks bible by Matt Lombard, The 2010 version would be adequate and you can get it 2nd hand for about £10 from amazon marketplace or ebay