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At the beginning of Avengers: Endgame,

Tony Stark is floating somewhere in space with Nebula. Suddenly, Captain Marvel appears to rescue them.

But how did she know where to go? Space is huge, they were nowhere near anything of significance, and they were out of contact with everyone. How was she able to show up in the right place?

She was sent by the remaining Avengers team on Earth, which includes Rocket at this point.

In the Captain Marvel post credit scene, she turned up in response to the beeper summons from Nick Fury before he dusted at the end of Infinity War. She asks where Fury is, and is then presumably filled in on all that has happened.

As Rocket is on Earth, he would most likely know that Quill, Mantis and Drax had headed to Titan to try and save Gamora in response to Nebula's summons (in Infinity War). He would have been able to tell Carol where to start looking for them, which gives a smaller search area for her to find them in.

It's also very likely that Nebula and Tony were broadcasting some sort of distress signal that could be picked up as soon as Carol got close enough.

The Avengers also refer to carrying out deep-space scans from Earth in Endgame, so they must have some sort of technology to allow them to try and pick up these signals - perhaps Rocket helped them build something to help find Thanos.

Since Carol Danvers apparently has the ability to detect Fury's beeper/beacon from across the galaxy (irrespective of light speed limitations, as shown in the mid- or post-credits scene in her own movie), I don't think it's all that far fetched that she can detect a distress beacon from Quill's ship after Stark and Nebula managed to get the power cells working well enough to get off Titan.

Both technologies are designed for FTL communication with similar information levels, and both are apparently within either Carol's powers to detect, or covered by Kree technology in her suit.

Captain Marvel was "summoned" to Earth by Nick Fury at the end of Avengers: Infinity War when he activates the pager that she gave to him. In the Captain Marvel post credits scene Carol arrives on Earth when the pager stops working and the Avengers still have their pre time jump looks. I believe this scene takes place before Carol rescues Tony and Nebula.

If that is indeed the case we can assume the Avengers gave a rough idea to Carol about where to find Tony. This is also plausible because we know the Avengers are tracking energy across the galaxy/universe as they find out where Thanos was hiding using the same technology in Avengers: Endgame.

Alternatively, Carol could have just run across them on her way back to Earth but this seems less likely as when she pops up to save them she seems to be doing it on a purpose, as if she was told to find them and didn't just stumble across them.

This is pure conjecture and not supported by any concrete evidence but my theory is that she followed a trail left by Maw's ship.

There's lots of people in the universe who would have known Thanos was from Titan but none of the Avengers or Rocket would have known for certain that that's where Maw was taking Strange. However, Bruce was present in New York when Strange, Tony and Spiderman fought Maw and he saw them enter the ship so he knew they were taken somewhere. Assuming those ships leave a detectable trail and Captain Marvel has the means to detect that trail (both plausible assumptions but I have no evidence to support either of them), she could have followed it and that would have eventually led her to Tony & company regardless of where they had gone.

And odds are, Tony and Nebula were headed back to Earth when they broke down so Carol would have run into them on her way to Titan, or at least come close enough to pick up their distress call.

It's likely that the post-credits "Where's Fury?" scene is canon. I expect there was a much more elaborate "get to know you / debrief" scene that was intended to be part of Endgame, but it proved to be fairly dull and was cut for time.

Rocket absolutely knew "the morons" were going to Titan I, and would have mentioned this to Danvers. We know Danvers is pretty good at planetary surveying, so probably would've taken a pass around Titan. She would have found the fresh disturbance of a fight scene, and the landing feet impresson of the Benatar (and no ship) and possibly a puddle underneath it. Where did it go? Where would it have gone?

From that it's a matter of either following whatever trail a ship may leave (especially a damaged ship leaking stuff). Or simply guessing that they would make a beeline back to Earth, and following that path.

Or following a distress beacon - you have to figure if today's aircraft black boxes have a 30 day battery to power a sonar pinger, starship distress beacons have an independent power source too.

Also, Titan is hardly "not anything of significance", Thanos' homeworld matters. Tony was joking about the nearby 7-11's; I'm sure the franchise in that part of the galaxy is Circle-K.