There is a topic with your question..
Here is:
scirra.com/forum/how-to-make-my-own-sprites_t123841
First you need to know, how to design something.. You can try with photoshop or illustrator for vector sprites.. There are a lot of tutorials on youtube, google, etc
If you don't know what is the "design".. Need to learn about, then learn the programs and if you 're ready try to create you own sprites, etc
Cheers