A warrior is a man who has demonstrated exceptional feats in combat.
Yes, a man, because there never were female warriors. Simple biology really, as well as historical facts. And yes, actual battle engagement is also required to even be considered a warrior. But mere combat is not enough to be deemed a warrior. An element of exception is indeed always involved.
As Ἡράκλειτος (535-475 BCE) put it:
Out of every one hundred men,
ten shouldn’t even be there,
eighty are just targets,
nine are the real fighters,
and we are lucky to have them,
for they make the battle.
Ah, but the one, one is a warrior,
and he will bring the others back.
In other words, being a warrior requires mastery of the art of war, as well as mastery of oneself, and pretty epic achievements.
As a man, however, you write your own destiny. The length of your life is always decided by Urðr (the eldest of the 3 Nornir), but how you spend the time allocated to you on Miðgarðr (earth), and what deeds Verðandi will weave in the tapestry of your life, are entirely up to you. Ultimately, Skuld decides of your afterlife based on your actual accomplishments, and in the case of the various halls for warriors, your combat record. But whether you are or will ever be a warrior is on you.
Half of the very, absolute best warriors who die in battle go to Glaðsheimr in Valhöll with Óðinn. The other half goes to Sessrúmnir in Fólkvangr with Freyja. The remaining excellent warriors join Týr at his hall, Valaskjálf. Finally, soldiers (including sailors and Marines) who have still experienced combat but have not reached the level of a warrior, go to Bilskirnir, Þórr’s hall in Þrúðvangar.
In practical terms, this means that the overwhelming majority of military dudes will never reach Valhöll or Fólkvangr. Most operators and fine United States Marines, though, will party with Týr at Valaskjálf. All other good servicemen will raise Hel (well, not literally), with Þórr in Þrúðvangar.