You'll arrive and find a geezer guarding a deserted gas station. Don't annoy him for now.
Now there are four ways into Navarro.
First way, the "heroic" way, is to head north, guns blazing. Personally, I wouldn't advise it, since first thing you run into is three guards in advanced power armour with plasma rifles, and two turrets with dual plasma cannons, and by the time you finish them, the whole rest of the compound wants to get into the act.
Second way is going west, disarming the traps, and down through the
vent. Use the blue keycard from Gecko on the door there, and get some
xp for releasing the captive sentient deathclaw. This approach does
have its merits, but the bad part is that after that, the deathclaw
will run, and you won't have him by your side. He's a tough one, and,
well, you can use all the help you can get if you want to do the
battles for the kill xp.
Third option, and my favourite, is to kill the gas station attendant,
then go through a manhole in a building nearby. Make your way into
the commander's office, kill him, get the Tanker Fob. Then find and
loot the Advanced Power Armour in a room full of lockers. Give your
old Hardened Power Armour to someone else in your group, like Vic or
Sulik, particularly if you want to do the ground level, too. Kill
everyone else on this level, except the deathclaw. The deathclaw will
join you, but only as long as you don't open the door that requires
the blue keycard. At that point he runs away. So if you don't open
it until after you do the surface fights, you have one extra helping
hand for free. (Though if you get him killed, you no longer get the
quest xp for freeing him.)
Now if you feel tough enough, you can go upstairs, on those circular
elevators and clear the surface, too. If not, hell, you can finish
the game anyway, but you'll be missing hardened power armours for
your group at the Enclave battles.
On the surface, there are a couple of things you can take, and which
are basically the sole reason for bothering with it. The vertibird
plans, an electrical engine and a K-9 motivator (looks just like the
engine). Be sure not to miss the plans.
Now what may not be obvious is that you don't have to clear the
surface in one go. Get up there, and stay relatively close to an
elevator. Shoot as much as you feel safe, then use the circular
elevators to go down and heal, if you or any of your group starts
taking too much damage. Also, you can skip some of the guards and
turrets, if you can get the plans and engines without triggering them.
E.g., the front gate guards probably won't come after you if you stay
around the back area.
If you want to, you can use the K-9 motivator to activate the
cyber-dog downstairs and take him with you. He's just the same crappy
cyber-dog as the one from the NCR scientist, only this one can also
talk. Other than that, it's still just as crappy in combat, and you
still can't use it to carry excess weight.
Fourth way is to do it "on the sly", if you've talked to the
Brotherhood of Steel guard in San Francisco and he's told you that
Navarro is accepting recruits. Again, don't leave your companions in
San Francisco, if you choose this approach. Just ask if this is
Navarro, tell the gas station dude that you're a recruit and you hired
an escort, find the password. Go north. Tell your companions to wait
you there, talk to the guard in front of the gate, say the pasword.
Do NOT talk to the drill sergeant, as you're told at the gate, or
you'll get assigned to a post (not necessarily bad), and the sergeat
will start going around in circles, checking all guards, including you
(bad, I think, tho I always managed to avoid the son-of-a-bitch well
Time to get yourself an advanced power armour. Go downstairs, find
the quartermaster's office in the NW corner, bypass him and loot the
lockers in the next room. (Alternately, if you actually want to
role-play getting your armour, remove your power armour and wear
something else. Go talk to the cook, in the large building to the
west of the gate. You'll get told to go downstairs and get outfitted.
Go downstairs, look for quartermaster, say you're new, get told to go
get your power armour and weapon from the next room.)
Go back, talk to the cook, and ask what's happening around, where's
the main base, and about the fob. Go downstairs, find a closed room
with a guy in power armour inside (the commander), a couple of
bookcases, and a couple of lockers. And a guard at the door. Talk to
the guard, BS him that the base wants the FOB secured. Go inside,
talk to the commander, open the locker and get the FOB.
Go back upstairs. Now I'm told that you can talk to Raoul, and then
BS Quincy that Raoul sent you for the plans. Didn't work for me, so I
had to sneak and loot the locker behind Quincy. If you also want the
engine and the motivator from behind Raoul, sneak there, too. These
mechanics get jumpy if they see someone looting their lockers :)
Or you can use a mixed approach. E.g., since the toughest battles are
on the ground floor, you can use the sly approach just for the that
floor (vertibird plans), then get your companions, kill the gas
station attendant, go down and shoot your way through the underground
floor (for the Advanced Power Armour and Tanker FOB.)
Either way, don't forget to open the door downstairs to release the
deathclaw, when you're done. That's worth 1500xp.