Free advice is overpriced, but here goes:
. At 300 lbs and 6'2", start with a doctor's checkup to make sure you're fit enough.
Once you get that go-ahead:
. Start doing off-ice exercises daily, in your case aerobic exercise is just as important as core strength.
. Balance and Core videos or video games are good off-ice workouts a few times each week.
. Get your own skates and skate weekly (or more) for at least 8 times to make sure you like it.
. Get the skates sharpened before use and store with dry soakers on them.
. Use the hard rubber guards when you're walking around.
. Dry the skates and blades well before putting the soakers on for storage.
. Most adults just use an old carryon suitcase or a backpack/tote for skates.
. Take the group lessons to start out - they're your most economic option.
. If you have a choice, go with the Basic 1-8 track instead of the Adult track.
. Print out the
curriculum and keep it in your skate bag.
. In addition to the lesson, plan on spending 2-3 additional hours each week skating and practicing the elements on your curriculum sheet.
I suspect the reason you're struggling is because of your weight - you've most likely grown in both height and weight since you were little, so your center of gravity and strength have changed. You have to regain your strength and balance, which will help you lose weight, but you have make sure you're eating properly and at the right times.
Bring a snack to eat after you skate - something healthy - to prevent Snack Bar Temptation.
As for what skates to get, it's tricky. I would suspect that you have a wide foot and there aren't that many choices of men's skates with wide widths. Most pro shops aren't carrying stock in adult sizes because of the economy, so finding a place to try on skates is difficult. There aren't a lot of men's used skates for sale regularly.
Start by finding out the correct size for both Riedell and Jackson skates - they have separate measuring sticks. The pro shop can measure you and recommend a model. Given your size, you need a pair of skates with good support, so you don't break them down too quickly. All-PVC models cannot support your weight - they'd be like socks within a few weeks.
Jackson has a men's model called the "Marquis," but they only come in one width.
You could pay a little extra and order a wide-width Classique in black.
Both have PVC-bottoms and leather/vinyl uppers, so they're low-maintenance.
If not, the Jackson Freestyles are the next step up, but they're more expensive and have a bigger blade which might be too much for a beginner.
Riedell models are the same: the Men's Riedell 121 only comes in a "Medium" width, but the white version comes in Medium and Wide. For a price, they could order the right color for you. You can just wear them as-is (white) or put on a pair of black boot covers. The next step up is the 133TS, but it's a little more pricey and has a freestyle blade on it, which will take some getting used to after wearing oversharpened rentals.
That's my 2ยข worth - good luck and welcome to skatingforums!