Plenty of good advice, but my experience is that shedding some weight has the biggest impact on hillclimbing, I notice every kg lost which is great motivation to lose another...
I've never been good at on the bike training, I think I'm too easily distracted, or just enjoying the ride. If I have a specific goal I do better using the gym, I probably get more from an hour spin or wattbike session than from a days riding, though it's a different sort of fun. If I can fit in two a week, it's not many weeks before I notice the difference on the bike.