Humans are one of the easiest species to roleplay, but a number of common mistakes pop up when playing one in the Star Trek universe. The foremost of these is remembering that humanity has undergone a massive shift in consience. First Contact sparked a drive to explore instead of war with each other, when the long-supressed urge to reach for the stars was finally made possible. Hatred, discrimination, and ignorance have all but disappeared in the centuries since Zefram Cochrane's historic flight. No longer does the color of one's skin, or gender, or even point of origin matter.
Another common trend is for all humans to be from Earth. To quote James T Kirk, “We're on a thousand planets and spreading out.” That's a lot of planets that a human could come from, and more have been colonized since the quote. Humanity is spread across the galaxy, and it's always seek more places to claim. Colonies often differ from the homeworld, as people leave major worlds for various reasons. Some leave due to ideological differences, some for the chance to claim a planet for their own, and some for the challenge of building a civilization from almost nothing. The earliest human colonies were on Mars, Alpha Centauri, and a handful of nearby planets. And yet, with all these options, people still choose to come from Earth.
Another common trend is to play them as if humans in the 24th century are as warlike as we in the early 21st. As mentioned earlier, human society has mostly outgrown the need for war, and resources are plentiful. Ideological differences usually don't pose a problem, as humans are more open-minded than they are now, and if tension does grow almost unbearable, the 'splinter' group often leaves to form a colony of it's own.
Lastly, rather an opposite of the last point, many people overcompensate for the fading importance of war, and are purely peacefulness. Hostility is fading, but not gone, and humanity still recognizes that, in some circumstances, correctly applied force is still the best solution. It simply alters the application of such force from overwhelming force and all-out war to surgical strikes, and commando raids, designed to cripple the enemy. Federation units dedicated to combat are usually trained at the special-forces level of today, and serve roles similar to that of the SEALs, US Rangers, and SAS.