Hi and welcome to 3FC!
It's awesome you're ready to make a change in your life. Stay here during the good times and the the bad and we will help you through!
At my HW of 307 I was finding it difficult to walk long distances without experiencing back-pain and I am 23! Things are much, much easier now, so I can tell you it doesn't take long. Healthy, nutritious food, regular exercise (basically, just
move! Doesn't matter what it is!) and a few pounds lost and you'll be feeling pretty wonderful.
Good luck, and I hope to see you around!