For undereye circles, I apply concealer with a brush, covering the area I want to cover. Then, I pat the concealer into my skin until its blended.
For blemishes, I use a brush to apply it only on the zit, and pat to make the line less harsh..
I've seen some of your other posts (your tagname is memorable), so I know you are young. You probably won't need to cover much else.
For more information on concealing and concealers, read one of Bobbi Brown's books, especially the Makeup Manual.
Hope this helps!