Nothing much more to add than the excellent advice allready given .. You may be able to see any dirt  or hairs on the mirror which will show up on the viewfinder, but on the sensor not so easy... and you will need to look at the menu function on the camera to open access to the sensor by flipping back the mirror,  to allow the blower brush John recommended  to be used, make sure you never touch the sensor with it.. .  And for sure if you dont know what you are doing take it to a dealer... The best way is to have good practices and not change the lens in less than ideal situations if possible indoors.. Not out in  dusty conditions..

 One way you may tell if dirt is on the sensor is by taking an image of a sky or sheet of white paper... if dark spots show up there in the same place it is on your sensor.. Hold the camera without the lens facing downwards and blow up so that any dust may fall out if you are lucky..

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt