Someone help me with Tumblr?

I need some help, i just recently set myself up with a tumblr account. Now i havnt even made a post as yet because i really havew no idea how to go about it. So please let me know how to write text and post pictures. Also is there unwritten rules for tumblr eg. if you come across a picture or something that you like on a randoms persons tumblr are you allowed to save the picture and post it on your tumblr if that person isnt following you? I'm so clueless to how to go about it? Can you reblog other peoples picturs if you dont know them?