How to prepare for Rathalos the first time
If you've taken on Odogaron as part of the Assigned quests (Rathalos and Diablos are the encounters after that) then you should be ready in terms of armour and offence. That said, upgrading your armour - whether through using Armour Spheres for what you have, or farming a new set from one of the recent monsters - won't hurt.

Rathalos can give you poison, so be sure to have a full compliment of Antidotes. Mega Potions are also essential. We'd also recommend unlocking the Area 1campsite, which you'll encounter on the way to completing the area's Grimalkynes quest, since Rathalos spawns and returns to Area 1regularly.
There are also a Tailrider camp very close by (again, you'll find this part of the Grimalkynes quest) so you could start your hunt to recruit one of those, giving Rathalos another target to be distracted by.
Rathalos location
You have to hunt a Rathalos as part of the Assigned (main story) quests, after you seek the help of a First Wyverian. - If you remember back to that quest, the Rathalos will spawn in exactly the same spot - in area 1- just south of the Area 1camp.
If you haven't already, unlock that camp (you'll do so as part of the Grimalkynes quest), allowing you to start fighting Rathalos within 30 seconds of starting the hunt.
Rathalos weaknesses

You should approach Rathalos like every flying beast you've encountered so far. When it's in flight, keep moving at a distance and circling round. Unlike Anjanath where its fire attacks were deadly, when Rathalos breathes fire its well telegraphed and either directly ahead or around it, making it easy to avoid.
The problem is its swoops. It'll very suddenly dive towards you, and if you are caught by its talons, you'll be poisoned. You'll want to use an Antidote as soon as you can, so hopefully your Palico and a Tailrider will distract it long enough.
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After certain swoops it'll hover momentarily, with its tail just above the ground, giving you enough to get a melee strike in if you are close enough. Otherwise, we'd play a war of attrition here. Forget all notion of attacks when its in flight, and wait until it settles on the ground.

When Rathalos lands, here's when we recommend you attack. Again, watch its head; if looks at you, it's about to turn around with a tail swipe, so roll away. The most frustrating attack is a little hover; if it strikes you with its talons, you'll get poisoned.
As with any monster, the key here is patience, and attacking at the end of its own attack, then ducking out of the way and repeating.
Rathalos has the annoying habit of flying far away from your location after a short battle. Use your various camps to your advantage (again, unlocking the area 1camp will help here) to fast travel to its nearest location, and recruit Tailriders along the way if its easy to do so..