You already own this bike and might want to put off buying a replacement for a while, hopefully when you're below 200-200#s and have more options.
This bike can be made suitable for your needs for little dough, and though it may still be less than ideal, it'll be fine for now. The most important change would be high pressure smooth tread tires suited to pavements.
1- the bike already has all the gearing you could conceivably need to climb, the rest is a matter for your engine, handicapped by the high payload. shaving a few pounds off a bike won't get you up hills any easier, buy shaving another 25#s off your body is worth 10%, and in the meantime your engine will (hopefully) get more powerful. The real key to being to climb hills is to climb them. Find hills you can manage, and incorporate as many as you can into 1/3rd of your rides, ie. a long flat ride, a short, fast flat ride, and a hilly ride.
Knee and wrist pain may relate to your fit, and you might need some help form someone who can see you on the bike. The basics include a saddle high enough that the knees are almost straight, but not so at the bottom of the pedal stroke. The wrist wants a natural hand position with the wrist not bent back or rotated. The knees also benefit form lower loads and faster cadence. Most newbies don't shift low enough or soon enough when climbing, and this strains the knee, especially when pulling a heavy rider up a hill. You have low gears --- use them
As for learning mechanical work, the best bet is a repair class offered by shops or some adult education programs. If there's a
bike co-op near you, that's a great place to start.