To place the the armies for 1st time, you must wait your turn. Once you are given the turn, you must place 5 armies in your countries. After that, send them to the server clicking on send armies button. Then, you must wait again for your turn, and place another 3 armies in your countries. You can place all the armies in one country if you want (there is no limit on the number of armies per country). Once all the players had placed the 8 armies, you will be able to attack.