Battle: Los Angeles is an excellent documentary on the topic.  

Pretty much this.

1. "They aren't out there" to begin with. The Universe is far too large for us to be 100% technically alone, but Humans are very early to the game and anything remotely like us is exceedingly rare. We are functionally alone.

Anybody or anything remotely like us, sentient/sapient with technology, whether they're a million years behind, and just knapping flints, or a million years ahead, and playing billiards with black holes, they're probably a dozen + galaxies away or further. The light from the Milky Way they'd see "now" and us seeing vice-versa, left when trilobites were a thing.

Give it 500,000,000 years, to a billion for a 50/50 shot for anybody to meet up. We'll either be extinct, or we'll be the "ancient space-ghost aliens not to be fucked with."

2. We have nothing they want. There's nothing on the Periodic Table we have they don't. Water? There's more than Earth's oceans on one gas giant ice moon in our Solar System alone. And there's fuck-all we could do about it if they took it, or hoovered up Saturn's rings amd gave us the finger.

And of course, umpteen Trillion star systems with water ice, metals, and whatever you want with nobody home at all. That are also way way closer and far more convenient.

Biological? Unlikely. Go to the bottom of the ocean and fry up the weirdest-ass nasty sea-slug, blind fish, or hydrogen sulfide worm by a geothermal vent for dinner. Despite the obvious issue of being disgusting as fuck, it might even kill you. And this is your home planet.

Something from another star system? That's all 1000x worse.

Women? Oh, so now you're gonna fuck the sea-slug?

Slaves? As if the robots a species that crosses interstellar space aren't perfect already?

3. There's extermination, friends, or secretly study. Those are the only three possible motives. Extermination, easy, smack us with a fast rock.

(Avatar? Earth & the RDA are really that mean & greedy? Second ship to Pandora lets a connex box of garbage go at 60% light speed right before flip-over & deceleration.  

"What Na'Vi? 9ft tall creepy-sexy cat aliens with tree-internet? Hey... uh, SECURITY!  SECURITY!"

And the RDA mines all the schweet Unobtanium ore from that asteroid field oprbiting Polyphemus in the Alpha Centauri system.)

Friends, say "Hi" and compare notes on the Universe & physics. Secret study, a preliminary step for extermination or friends. Although again, we've got better PowerBall odds than any contact at all to make these scenarios  possible whatsoever.

Ok, that out of the way, how do you defend against aliens? The imaginary Hollywood stupid ones that cross interstellar space, get in low Earth orbit or hang out in cislunar space, and then use rayguns & bombs onesy twosey on us? Be that fighter planes, cities, or individual people running around?

Operation Plumbob manhole cover. Google that.

Except we optimize it, a bunch of tungsten balls, whatever size is best for velocity retention & atmospheric drag, vs. shotgun spread & hit probability.

Some sort of "wad cup" and the borehole/barrel is filled with lightweight polystyrene or something similar that will vaporize nicely, producing good high-expansion coefficient plasma for the shot.

20-50 kt nuke at the bottom, and... Bob's your uncle.

The tungsten shot, baseball size, bowling ball size, whatever works best, will be moving at well over 66 km/s, better than Earth escape velocity, better than Solar System escape velocity. I think... that might even be substantially higher than Milky Way Galaxy escape velocity...

I don't care if they've crossed interstellar space, relativistic or FTL, whatever it is, won't spool up and switch on at the "speed of plot."

Shields... shields like Star Wars & Star Trek, just not a thing. If the alien invasion fleet got here "the hard way" via relativistic travel, they'll have a relativistic dust shield, big-ass umbrella thing, because a grain of sand hits like a nuke at 80%+ of light speed. Or it might even just be fog or snow they spray gently ahead of the ship thousands of miles ahead of them.

But "Mr. Brunswick made of Wolfram" masses a fuckton more, and won't vaporize like the sand grain will.

A bowling ball chunk o' tungsten will mass about 105kg. Say it's moving at 100 km/s because it was intentional rather than an "oopsie" like Op. Plumbob...

That's 525000000000 Joules or 387,219,000,000 ft/lbs, since we're an American gun forum.