1. Always look for the copyright first. You cannot use somebodys work without his/her permission!
2. For your first games and for prototypes go to the site of opengameart.org
On this site have lots of free graphics you can use in your games (still you must agree to the licence, e.g. put the
name of the artist in your game). Most images there already come with transparency ready for game making.
3. To remove backgrounds in images use gimp, you can download for free. There are many tutorials in the internet.
I clean the black bg in the player image in gimp in 1 minute (look attachment).