first, start here:
http://www.dol.wa.gov/driverslicense/cdl.html
to drive class a, you must pass the combination vehicles exam for your
state, as well as general knowledge and air brakes. since you already have a
b license with this, you will probably not have to retake it, but a refresher
isn't a bad thing, and there are plenty of free practice tests on the 'net.
you will also have to road test with the type of vehicle you intend to drive.
(this is where a school is very handy if you don't know someone with a
tractor trailer to practice backing with)
i don't yet have a doubles, tank or haz endorsement, but i will get them in
time. more license, more opportunities for more different types of work.
I went to Swift's school in phoenix, and just finished the 50 days of hell (also
known as mentor training). the CDL school can be done at varying levels of
cost, depending on your individual timetable, money to spend (or borrow),
and if you can adapt to the OTR lifestyle, which has been fairly easy for
me, but can be difficult for people with families.