(noob question) how do i create user and and make its right limited only

hi guys….recently i just finished installing wordpress on my website..
im totally new to computer things im just learning by reading and test it…so i have a blog that i planned to let visitor create an account on my blog and make a post with picture on it but after searching an hour or more on google i cant find the easier answer…how do i set my wordpress to let user create an account with limited its right just only to post in easier way to understand for noob(thats me :D)..and let only upload an pic jpg (my friend ttold me that if i let people upload file with no limited extention that my blog could get hacked)is there any plugin that let you do this?
thank…..