Im rather new at the game to, so my tactics may not be the best. I do appreciate all the great comments on this board though, reading through them has really helped me out.
as has been pointed out before, mines work great against the AI unless you want to be challenged
so what I have been doing is building strong weapon platforms on core planets and hauling them out to the frontier as it expands.
i like to load missiles on the WPs for range, but you need to change it up every so often and throw in direct fire weapons to take things down when they get close (and when a fleet has heavy point defense)
for defending warp points, I like to throw up 2 space stations (yeah, takes time. drop off mines or fighters in the short term) and you see some real synergy take place when their point defense systems work together taking out fighters or missiles. what one does not catch, the other will (mostly). I had 2 bases setup with null space guns and lots of point defense, hordes and hordes came at them but nothing ever got through. every so often i would drop off some disposable point defense or missile sats to help them out, but that was probably a waste of resources when you think about what mines or fighters could have done (or a third, smaller station).