For international tickets you will have to go to an international ticket office at one of the main offices, call the tele sales, or use the international sales website.
For many destinations it is possible to buy tickets online and print them out yourself. For others you can choose to pick them up at a station, or have them mailed. Open tickets, valid up to a month are however not sold for high speed trains. These are always for a particular train.
Tickets for domestic travel are valid for a whole day, and are open, in so far as that they are not specific for a particular train. If you have for example a return ticket Brussel - Brugge for May 5th you can take any train from Brussel to Brugge, and then any other train back again later. You can purchase tickets for use on later dates in advance, but an open ticket valid for a month is not sold for domestic travel.
If you plan on doing multiple trips in Belgium a 10 trip card, called a "Rail Pass" might be a good buy.