Don't start with campaing. Some missions are hard, but what is more important, they are all very confusing, with lot's of possible targets, lots of 'fail' conditions.
Start with single missions, based on number of stars by them. More stars, more difficult mission. Start with one platform (helo has no tma, kilo is simplest sub).
Get guides and more here:
www.subguru.com
There is also wiki somewhere here
http://www.commanders-academy.com/forum/index.php
but I can't find it at the moment
Let's wait for Oneshot, he will make it clear.