I think a very important part of any successful exercise regimen is to set concrete goals and challenging yourself to meet them. For example, you could set the goal of getting to a certain body weight or body fat percentage by some date, or run some distance in a certain time, or setup a competition with a buddy to squat some weight, or whatever. The result will be that going to the gym & doing cardio will feel more like training and less like painful exercise that you hate. Another great option is to try a program like
Crossfit which has a competitive aspect built in. The challenges there include outperforming others doing the same workout, but even more importantly, improving your performance on the same workout over time. Hell, even doing the WoD as "Rx'd" is going to push you to your limits. I personally find things of this nature VERY motivating.
As for getting a punching bag, it's a terrific form of cardio. As with most exercise, your results would be a lot better if you had proper instruction, but if you're careful, you can somewhat learn the basics on your own (as long as you use it
just to train for cardio and not for actual fighting

). As for hanging it on the I-beam, it depends how you do it. Out of stupidity, I had originally hung my bag by just wrapping the chains around the I-beam. The result was that the energy of every punch transferred through the chains, into the I-beam and shook the entire house. Moreover, the chains started to grind into the beam. If you instead drill it and securely attach a hook into it to hang the bag from, you'd probably have better luck... I got a crappy punching bag stand at sports authority which, when weighed down with four 50lbs sandbags, gets the job done.
In terms of the equipment itself, you'll want to get at least a heavy bag, wraps and gloves. The size of the bag depends on your own bodyweight and strength. I'm ~200lbs and hit pretty hard, so I got a 100lbs Everlast bag on sale. I've used it with some friends who were 160lbs and hit a bit lighter, and they still get the bag moving around quite a bit, so I wouldn't get anything too much smaller. Getting wraps and learning to use them - there are tutorials online and often on the box the wraps come in - is essential to protecting your wrist. The gloves protect your fingers and knuckles. Neither should be considered optional.
The final piece is the workout itself. Go online and watch some boxing and kickboxing tutorials and
start slow. Make sure you are punching fairly lightly until you get the technique down or you could easily hurt your wrist. Everyone will have the occasional "bad" punch that sends a twinge of pain (a nice reminder to watch your form!) but don't end up being that guy that shatters his wrist and spends the next 1.5 years healing. There are many workouts you can do on a heavy bag, depending on fitness. Most people break up their workout into rounds - go at the bag for 1-3 minutes and then rest for 30 seconds to a minute, repeat. You can focus certain rounds on various aspects - for example, do one round where speed is the emphasis, another where you work on multi punch combos, another where you develop punching power, and so on. When I workout with friends, we do a "Simon Says" style workout where one person yells out punch combos ("One two!" = jab, cross) and the other executes them as quickly as possible.