Most of readers' information concerning Mirkwood comes fromThe Hobbit.   In this novel, the dwarves and Bilbo cross through Mirkwood on their way to the Lonely Mountain. 

Before the Third Age, Mirkwood was known as 'Greenwood the Great,' but Sauron secretly built a fortress in Dol Guldur (a rocky hill near Mirkwood), and the evil shadow slowly crept into the forest, imbuing it with dark creatures like orcs and humongous spiders. 

There is an old road that runs through Mirkwood and an Enchanted River, whose dark waters will make any trespasser fall into an uneasy sleep.  InThe Hobbit,Bilbo finds entering the forest a foreboding prospect:

"The entrance to the [forest-]path was like a sort of arch leading in to a gloomy tunnel made by two great trees that leant together, too old and strangled with ivy to bear more than a few blackened leaves. The path itself was narrow and wound in and out among the trunks. Soon the light at the gate was like a little bright hole far behind, and the quiet was so deep that their feet seemed to thump along while all the trees leaned over them and listened."  (from The Hobbit )