Several good choke points are north and south of Switzerland, the western end of the Med, Italy and the middle part of the Med, and the obvious stalemate line around Prussia, Tyrolia, Sileisia, Bohemia.
There are many more, and it depends if you are fighting armies or fleets of course.
As for advice for new players, well, here are my top ten tips:
1.) If something looks too good to be true, it usually is. Always look a gift horse in the mouth!
2.) If someone is helping you, always ask yourself *why* What is their motive?
3.) Look for unorthodoxies. Do not be predictable, as most players play on and respond to predictions. Try and go for the less expected moves.
4.) Make use of non-supply centre spaces. They are there for manouvres, so use them! Occupying these first will unlock gains of centres later. Also, many players do not defend these spaces so well.
5.) Do not be afraid to give up a centre for a centre that puts you in a better position tactically.
6.) It's been said before, but I'll say it again. Always try and cover retreats by your opponents!
7.) Crushing an opponents unit is extremely valubale if their home centres are a long way away from the front line. It can often be the key to unlocking an apparent stalemate line.
8.) Talk. Talk some more, and keep talking. Even with players who have stabbed you and you have stabbed as well. Often, you may have to come to them cap in hand later in the game when the situation changes.
9.) If someone has stabbed you, do not immediately seek revenge. Ask why they did it, then look and see how best to counter it. Often, they will stab because someone else suggested it. 50% of the time you can stop a stab in its tracks by pointing out some simple facts.
10.) Never trust anyone. If you do, you get complacent and become an easy target.