I was about 260 when I started doing the 30 day shred with Jillian Michaels- it's on youtube for free. I highly recommend it.
Though, at that weight I spose we were about the same seeing how I am a few inches shorter than you. I started slow I made it about 8 minutes the first time including stopping a second so don't push yourself. I also couldn't do a jumping jack at the time so I did the arm movements and bounced.
I noticed a difference the first week. Don't go off my weight now I bounce back and forth between working out lol :/.
Maybe you don't need modifications but I definitely did. Try this if you want to start. Remember don't jump all in at once. I think it's great you are ready to get it going though, strength training is amazing and makes a huge difference in looks much quicker than on the scale.
<-- Here is some modifications.
This is me using mostly just 30 day shred for about 4 months but maybe missing 3 or 4 weeks out of the months months. I could also do jumping jacks about a month into it! lol http://www.photobucket.com/Buttercup8587
The first pictures in the panties and sports bra I was a little over 250 in May and the last set in the shorts and sports bra was September. I did 30 day shred about 3 days a week and some zumba for cardio sometimes. I'm sure my results would be more amazing if I did it more and stuck with it. I think I may had taken a month off out of all that.
I started with Jillian and I'm back on the 30 day shred it does amazing things. Just be safe, because when I started I went full force trying it all and did pull my hamstring. Then I couldn't work again for days. So, keep in mind to do your best but listen to your body too