Hi, thanks for the question. :) For a beginner, I would suggest Unity. It's completely free to start developing, and the coding can be kept to a minimum. There are also a bunch of tutorials available on YouTube and other sites for both 2D and 3D development.
As for game artwork, it is okay to use other creators art, as long as you give them credit. If you are looking for an artist, you can look on sites like this, or even create a new topic in this forum. :) If there's one thing that years of Newgrounds games taught me, it's that stick man games can be just as fun as any other. I would focus on making the game fun to play, looks come later. ;)