Currently the only way I know to absolutely guarantee getting a room at the hotel is to
be the convention chairman. Other senior staff posts are also good, but being the chairman is best. For a first time attendee, I do not recommend this course of action.
(By reputation, Uncle Kage may also endorse avoiding this course of action.)
Alternatively, there is no guaranteed way to get a room. There are 2500-3000 people trying to grab a room, and there aren't that many places at the hotel. The EF registration server will get hammered in the first minute, and then the Estrel will get several thousand emails in the next five, and it's largely a first come first serve situation.
Effective tactics to increase your chances include:
1) Forming a team with 3 other people with the intention of sharing a room. (Though this sounds like it might not be possible for you.) This gives you 3x times the chance. If you chose this tactic I strongly advise requesting a TWIN room, which will give you two beds, rather than a double, which will give you just one. I would advise to ask the hotel for a third fold-away bed as soon as possible, as supplies for those have been limited in past years.
2) If you don't get a room yourself, try your luck in the Room Sharing section of the forum. Remember to advertise you want a room, and to apply to anyone saying they are looking for a room mate. Check regularly and apply to every offer until you succeed, as if you were getting a job.
2) Booking a contingency room at a nearby hotel on the chance that you don't get a room at the convention site. Unfortunately this is something best done back in August, but there are still a few when I looked for a friend last week. If you do this, remember to budget for breakfast (included at the Estrel) and a ~30 euro 7-day A-B card for getting around Berlin, and specifically to and from your hotel and the con site. I'd also recommend bringing a rocksack and a large bottle of water. Remember to plan ahead if you want to buy anything, and especially anything large and/or expansive - as it will no longer be a short walk from the dealers den or art show to your room. Ditto if you want to transport bulky items to the convention such as art for sale or a fursuit. A taxi might be worth the cost to avoid the hassle and risk of damage.
If you haven't seen it yet, the Introductions sub-board may be of interest:
https://forum.eurofurence.org/index.php/board,34.0.html