To attack with a weapon skill, point the mouse cursor at an enemy and press the corresponding button displayed under the skill on the skill bar. (From left to right: Left mouse button, Right mouse button, 1 and 2 on the keyboard.)

Your combat capabilities are mainly determined by your equipped weapon. One-handed weapons grant two weapon skills each, two-handed weapons provide four. 

You can see what skills you have available on the skill bar at the bottom of the screen. Hover the mouse cursor over the skill icons to see a description of what each skill does.

Ranged weapons allow you to attack enemies from a distance and use terrain and cover to your advantage.