As a general rule your are more efficient seated than standing. That being said standing can be fun when you really want to get up fast, but be prepared to feel it. A situation where I think it makes sense to stand is if you have a short particularly steep hill, that way you don't loose too much speed and don't totally blow up.
As far as getting up hills, don't be afraid to down shift and find a pace that works for you. The hills on this ride really sucked for me, as you can see the second one I average 4.9 mph getting up it, but I got up it. Cadence was in the 50-60 range quite a bit, not exactly where I'd like to be but with my gearing that was what it took.
http://app.strava.com/rides/8811733
The best way to get better at hills is to ride more hills, learn what your body can do and push it further than you think you can (for me the first time I rode big hills I got to the point where I thought I had to stop but I said no I'm going to keep going, and I did).