I worry that you won't find too much help if you don't specify your budget, musical preferences (genres and signature, like if you like more or less bass).
Ohm is a unit of electrical resistance. It tell us how much current (flow of electrons) goes with given voltage (force that pushes these electrons). In realm of headphones that means how much power amplifier will be able to put into them, which is exchanged into sound by headphone drivers. Usually amplifiers can put more power into lower impedance drivers (like 30ohm), but that's not an absolute rule, for example some tube amps can put more power into high impedances like 600ohms. Also some people argue that higher impedance will give you better sound quality (better damping of resonances), so as long as you have enough power, it's better to aim for highest headphone impedance, and lowest amplifier output impedance.