Read/watch about the physics, know everything about that by heart, read/watch about the weapons and strategies, know those by heart, etc. The best way at that point to learn is read or watch every single instructional video you can find. If you miss this opportunity, the learning curve gets much steeper for the game for you and you'll have to struggle much more with it, making you less tempted to play it (which is why high learning curve shooters are probably not too popular anymore). Of course, this issue can be somewhat solved with ranked matchmaking systems as long as the playerbase is large enough for that (SC2 I think has such a tiered system, quake live and T:A have somewhat rank based systems as well). This allows you to be on a level playing field with the rest of the people and not be forced to play with more experienced people that dominate you as much. The problem here is that you have to have two things happen to you to be able to jump on the wagon of the game you want to play and not have it be too difficult of a learning curve: You need to be there when it hits and you need to have a solid skill for the mechanics of the genre when the game hits. Sorry if this is a bit rambling and incomprehensible, I wrote this at around 1AM after a few matches of Nexuiz. It's altogether a different game, not one I like, but ones that others like. Many of the weapons were also either tweaked or completely reworked, mostly giving the game a more sort of 'casual' feel, which doesn't draw from the same demographic as Xonotic does, despite them branching from the same game. This makes maps smaller, but the maps already had to be smaller to accommodate for the missing laser, which results in pretty damn small maps. This lowers the entry level and general skill ceiling, as any player can get an automatic Strength, infinite ammo, or invincibility, which boils down to simple luck of the draw. Nexuiz has 'random mutators' which dynamically change the game. This considerably lowers the skill ceiling right out of the gate, as part of the skill involved in Xonotic is learning to work the laser to engage and move around. The laser was basically their answer to rocket jumping, as a low-damage high-knockback weapon that also could be used on the self. The laser has completely been removed in Nexuiz.

There are a few key differences between Xonotic and Nexuiz:

Then, through some details I'm not exactly clear on, the rights somehow got sold to Illfonic to make the game now known as Nexuiz, and the developers of the original Nexuiz made their own little fork/remake known as Xonotic.

The gist of it is that Nexuiz used to be an open source arena shooter based off of the Quake engine, one that I happened to enjoy quite a bit. For the uninitiated, Nexuiz and Xonotic are intertwined in a very close way.

Nexuiz full screen