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, 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 soldiers, your combat record, as well as your blood line in some cases. But whether you are or will ever be a warrior is on you. And where you end up in the afterlife, is typically based on your achievements, under your direct control (with some exceptions).
First, some of the absolute best warriors, whether or not they die in battle, including Úlfhéðnar and Berserkir under their command, join Týr at his hall, Valaskjálf. Those don’t go to Valhöll. Ever. No matter what. This is over some unresolved issues between Blendingar (hybrids, half man half jötunn) and Óðinn (who formed the world by killing one of their ancestors through deceit and dismembering him, and later plotted to drown all of their ancestors in blood), and the concern the hybrids would side with Jötnar during Ragnarøkr, not to mention seek some conflict with Óðinn. Í vegi jarðar. That is the way of things.
The remaining 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. 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.
Ultimately, Valhöll is for those true warriors, the absolute best of the best, who actually die in combat, and who are not meant to join any other hall as it is the case with Úlfhéðnar. It is a great incentive for any man to strive to achieve greatness in combat.