While the Star Trek universe contains literally hundreds of alien species, there are thousands more we've never seen.  We encourage you to try one of the established races first for your character (and if you, be consistent with what's been established--research your choice).  But, if creating aliens is your forte, have at it--we'll welcome getting to know them.  However, be mindful of these guidelines:

    --  Too often "aliens" are merely humans in alien clothing.  If your character talks, sounds and acts human, and likes human things (things YOU like), then he, she or it should probably just be human.  (This doesn't mean humans can't be interesting.  If you want to show significant cultural differences, you might, for example, have them come from a colony world.)

    --  Likewise, aliens need not look like humans.  When Star Trek episodes showed aliens who look just like humans, it was often the result of makeup budget limits.  Humanoids are obviously plentiful in ST and the rest of the sci-fi genre, but again, this is a budgetary concern.  Many ST novels (and episodes) feature aliens who are not humanoid--and in real life, it's slim or unlikely that alien life-forms "out there" will be humanoid (much less resemble humans).  Since we aren't constrained by a budget, your imagination should not be constrained either.

    --  Hence, if your character truly is alien, then go all the way.  BE that alien.  Consider the various factors: The way he/she/it eats, breathes, thinks, dresses (or maybe they don't dress), walks, moves, speaks (how do they talk or convey information? what is their speech pattern? do they have mouths? how are they shaped?)... their physical form and biology--do they have different genders? how many?... their culture, evolution, government, industry, economy... How they view other life-forms, their relationship with Starfleet and the Federation, their understanding of even the simplest customs which humans take for granted... All will be different from what we expect of other humans.  Take this into account if creating a new race, and base your approach on it.  Be sure it makes sense (i.e., consistency) and be ready to commit to portraying such a character on a recurring basis.  Don't be "wild" or too alien (they have to be able to relate to other characters) or do it for shock value.  It should still be a believable character with believable motives and feelings.  As with any character, consider his/her/its life--how they did at the Academy, what affected or changed the character as a result of previous events, and so on.  All of this will add up to a "living, breathing person"... and an interesting character.

    If you need help or have any questions, contact the GM.