For the immediate time being, get your diet in order and just go for a daily walk. You can probably get your first 20-30 lbs with that alone. At the same time, you can go to the gym a couple times a week to learn how to lift or even just do some bodyweight stuff to prepare for the gym. Starting Strength by Mark Rippetoe has a lot of detail on how to perform the main lifts. From there you can pick a beginner program (Starting Strength, Stronglifts, Greyskull, or something else), and begin getting stronger while losing weight. This will only last a short time, though before the lifting gets more demanding and you have to decide on whether strength or fatloss is your goal.
After the initial water weight (5-10 lbs) is gone, try to lose 3 or so pounds per week for the first month. After that, you can taper it down to 2. Also, wrap a string around your waist, just below your bellybutton and cut it. You should be able to shorten that string every week.
You may also want to take a little break once you get around 250 or so and just hold that for a couple weeks to give your metabolism a rest. If you do the math, you’re looking at nearly a year to get the weight off. And at 5’9," you still won’t be “ripped,” but you will be healthier and stronger.
For fat loss, the trick, oddly, is to do as little as possible while still meeting your fatloss goals. This allows you to step up your efforts as needed to maintain progress.