Putting skinnier tires on your cruiser isn't likely to get you much, especially going up hills. You'd definitely be better off with a lighter bike, and one with gears. You can certainly still stick with an upright bike - look for a hybrid style bike. If you like Electra, consider a Townie or an Amsterdam. They come in a variety of models with different gearing choices. Neither of those is going to be an ultra fast racer bike, but they will certainly be easier to get up hills than your cruiser.