Tickets started on sale two weeks in advance so it is difficult to confirm. Some routes are popular and can be sold out in hours. It is OK if you are flexable in your itinerary and don't care about better seats.

Most likely you will need the app to buy tickets. I still cannot register into the 12306 app even though I have a chinese phone number and bank account. End up that I used traval agent to get tickets. YMMV.