Hi Seproten, - well I've been going to Roland Garros each year for the last 10 years now, so I will TRY and answer your questions as best I can......
The majority of tickets are sold in advance through the French Tennis Federation (FFT), in March/April time. You send off for a form (which will be sent to you sometime in February), and then just send the form back to them with your cheque/credit card number and keep your fingers crossed. Good news, - I have always got some tickets this way without a problem, - bad news, you can only buy tickets for a maximum of 3 days by this method and have to stipulate your chosen court in advance, so you will not be able to get all the tickets you require this way. If there are any unsold tickets left after this ballot has taken place they will be are sold on the door at Roland Garros about a week before the tournament starts. Just queue up and hope they have something good left.
During the first week, once you are inside, it is sometimes possible to buy "returns" for other days on the main courts from a "returns" kiosk underneath the Suzanne Lenglen Court, - there are not usually many of these though.
Undoubtedly the easiest way to purchase tickets is from the "touts" at the gates on the day. If you leave it til shortly before play starts they will normally sell them to you at face value (or very little more), and I have NEVER seen anyone stopped or refused entry with one of these tickets. This also means you can buy a ticket for the "right" court, instead of having a Centre Court ticket when your favourite is playing elsewhere.
As you will tell from all this it is probably going to be impossible for you to have tickets for every day before you get to France, and I guess it is a really long journey for you to "risk" it, but there shouldn't be a problem. Another good trick, - if you do end up with tickets for a court you don't want, it is always possible to swop them, either with a "tout" outside, or just by talking to other fans in the queues outside or maybe in your Hotel if you are staying nearby. I have done this loads of times.
Anyway I hope this helps, - Roland Garros is a wonderful tournament and I'm sure you will love it
Of course I don't know who your own favourites are, but the FFT are pretty good at "sharing" the top players around, so you should get a chance to see everyone over the course of the fortnight. For example, last year I saw Juan-Carlos Ferrero playing on an outside court, and quite a few others as well. Practice courts are easily accessible too and you can eatch your favourites practicing with no restrictions and take photos without a problem.
Most of all - have fun, - may see you there!!