play all game modes and see which one you like. there's a little tab somewhere on the bottom of the song select where you can select your game mode. personally i only play standard, so i would only know stuff about the gamemode where you click the circles.
now, for your question, i'm assuming you mean the hard mode that comes with songs/beatmaps, and not what you find hard. those hard mode beatmaps/songs are usually 3 stars. in order to test yourself to see if you are ready, try playing it. if you're able to hit the first 10 circles while scoring either 100 or 300 points, i'd say you'd be able to manage it. if not, you gotta work a little harder. honestly, this goes for every new difficulty when you encounter.
have fun playing osu! maybe you will be top player??