Hills are your enemy; hills are your friend.
Find some. Do them. A lot.
The best way to get better at climbing hills is to climb hills.
Google up some info on hill climbing advice. There's a ton out there. How you position yourself in the saddle, what gear you pick, when to stand vs sit... there are a lot of different techniques that different riders use. Find some techniques, experiment with them. Figure out what works best for you. Then, experiment with others, too.... because on long hills or rides with lots of hills, you may want to switch it up some.