I've been on online dating websites for a few months now and it has been good to me, who has a hard time approaching people in real life. I can't quite provide much guidance to you though because the male experience is very different from that of women. There are more women than men on there and men tend to be more aggressive. Maybe the one thing that I've heard from women on this site is to avoid replying to men when you're not interested because they don't handle rejection gracefully.
But there's one thing that applies to both genders: be honest. First, be honest with yourself about what you're looking for. Then, be upfront about it on the website. I had to shed a few layers of embarrassment and shame before being able to do this. Sometimes, I felt I was writing the "wrong" thing from a socially acceptable (in my view) standpoint, but I knew it was truer to who I am and what I'm looking for.
Dating is a matching game. It's not about impressing people, nor arriving to the top of some pyramid. The selection criteria are arbitrary: some people like tall, curvy, patient, smoker, witty, strong-willed, submissive, etc people. Go after what you desire, then find someone who desires someone like you. You might think "Yes, but everyone wants a tall dark handsome man! I can't be asking for it too!" or something like that, but no. Our tastes are not universal and you shouldn't negotiate against yourself. I've been surprised myself at how much it paid off to be honest and go after what I desire.
Oh and don't worry too much about disappointing people. If you're not interested, the kindest thing you can do is to be clear about it.
Good luck. I hope it will bring you joy.
M39 D6 D3 (at S) S 2014-09 D 2016-09
"You can't start a fire sitting around, crying over a broken heart" - Bruce Springsteen.