I am flying for the first time on Monday to Denver. I am 23 and terrified. Any suggestions? or things I should know?

My biggest concern is that I won’t fit into the seat, which is ridiculous because I only weigh about 250 and I’m 5′11″ and I know that there are bigger people than me that fly…right?

Please don’t ridicule me about my size, I have a disorder that makes it hard to loose weight, I just want suggestions about airline travel…any do’s or don’ts would be wonderful…thank you!