Big problem! Everything is wrong!

I uploaded everything to my website, as the 5min installation told me to.
But whenever I go to /wp-login.php and enter my info and hit enter, I’m told the page doesn’t exist.
If I then reload the page /wp-admin I get to my dashboard.
But if I make any changes in options, users, whatever, I’m told by the browser that the pages don’t exist.
If I reload they appear, WTF?
Can anyone help! I have a heavy deadline hanging over me!

Ooh! And another small thing.
How do I add a picture in a post, without it being online, and entering the URL. I’m @ a macBook.. So I don’t really know. I’m a newbie, forgive! I tried an older version of WP, and there was an upload pictures popup or something, I had a filebrowser, and now it’s gone, but the “add/edit picture” symbol is the same..

Thank you all so much in advance!

J