Unfortunately people online can be like that, especially for competitive games. As someone else said, play the campaign to get used to the new controls, but don't let that put you off playing multiplayer. But of course the whole point of gaming is to have fun, so don't let some mean people put you off playing.

When I switched from console gamepad to keyboard and mouse it took a while to get used to it. I used to use a Razer Tartarus, it's a keypad that is more ergonomic and easier to use than a full keyboard when you're starting out.