How to make a race "easy" way (not a real tutorial, but rather if you want to get started/make it quick).
1) You pick a creature you want to use as a civilization creature. If you want to make a new one, do eet.
2) You go into entity_default.txt file and copy an entity file of humans if you want it to be friendly or goblins if you want it to be hostile.
3) You change ENTITY:PLAINS or whatever it is to ENTITY:YOURNAME, where YOURNAME is an unique name.
4) You change CREATURE:HUMAN or whatever it is to CREATURE:YOURCREATURE where YOURCREATURE is the creature ID you want.
5) You tweak their weapons/ethics/whatever. It should be pretty transparent if you look how other entities are set up.
A few things to consider:
a) Various WEAPON, ARMOR: etc. tokens define which items does your race use.
b) DEFAULT_SITE_TYPE defines which settlement type do they use (CAVE_DETAILED, CAVE, DARK_FORTRESS, CITY, TREE_CITY).
c) START_BIOME tells where do they appear, and various BIOME_SUPPORT shows where can they expand
http://df.magmawiki.com/index.php/Biomed) ETHICS are there mostly for worldgen, it defines their relations between neighbours.
There's much more but this should cover the basics.
Note that agressiveness of goblins is dictated by BABYSNATCHER token, and seasons when traders come for friendly civs is dictated by ACTIVE_SEASON.