Arms and Armour of the Mithril Knights Warrior's Guild
As written by our Guild Master, Lord Elbren

Henceforth, it shall be said that the Mithril Knights Warrior's Guild shall bear only arms and armour as described by the Master of Middle Earth, Professor JRR Tolkien in his Middle Earth tales, his Letters, and his commentary.

Ted Nasmith, (in Elbren's opinion), is the Tolkien artist who most accurately portrays arms and armour as described by JRR Tolkien.

In the Book of Lost Tales II, you will find EXCELLENT armour descriptions in the Tale of the Fall of Gondolin. WELL worth the read for any Middle Earth warrior!

What Isn't Middle Earth Arms and Armour?
Plate Armour
This is probably THE most commonly made mistake when one writes a character having armour in Middle Earth. Tolkien's descriptions include mail, (aka chain or ring mail), but nowhere does he describe plate armour. Exceptions would be vambraces, (ie Swan Knights of Dol Amroth) and helms, (vambraces are the same as bracers and are worn on the lower arm; helms, of course, are worn one's head :)
What Would Comprise Middle Earth Arms and Armour?
Mail
Tolkien describes shining mail quite often. Mail refers to what, today, is called chain or ring mail; though, "mail" by itself, in the Middle Ages, meant chain or ring mail. (In fact, "chain mail" or "ring mail" is a redundant phrase.)
Weapons
Swords, axes, spears, daggers, bows...these are all a part of Middle Earth. What is not ever mentioned by Tolkien? Crossbows, for instance, are not mentioned. Named weapons are a huge part of Middle Earth, which, of course, is very much based upon the early Middle Age custom of doing so.
Some descriptions of Middle Earth arms and armour, (from Joe Piela's essay):

The Elven guards in Caras Caldhon are said to be wearing grey mail.

The Lord of The Nazgul is wearing a mail hauberk and bearing a mace when he is fighting Eowyn and Merry. Merry got around the mail by stabbing with his Barrow-Blade up underneath the hauberk and into the back of the Nazgul's knee.

Denethor wore a mail coat night and day beneath his robes so that he would not become soft with age.

Boromir was equipped with a shield, sword, and helm, but had no mail coat since he was travelling afar and presumably wanted less weight for greater speed. If he had reached Minas Tirith he would no doubt have been equipped as Aragorn had been.

Upon Boromir's death, it was noted that the orcs he slew had been equipped with mail, helms, and shields. The orcs of the Misty Mountains among this group were armed with scimitars, short bows, etc. This was largely inferred by the comparison to the three Uruk-Hai from Isengard that he slew. These great orcs were armed with longbows and short, broad-bladed swords, instead of the conventional equipment. At this point it was noted that the Orcs from Isengard bore white hands on their shields, and white S-Runes on their helms, while Orcs from Mordor used the sign of the Red Eye on their equipment. It's not clear if the Orcs of the Red Eye took part in the battle with Boromir. It seems more likely that they caught up with the other orcs a bit later on, so the comparisons in equipment seem to be made through the extensive knowledge of Aragorn. During the expedition of the mixed group of orcs across the Emyn Muil and Rohan, some notes are made referring to their hideous, jagged knives. Grishnakh, the leader of the Orcs from Mordor, bore and interesting knife whose hilt was carved in the likeness of a head with a vicious face.

In an essay in Unfinished Tales about The Battles at the Fords of Isen, some other pertinent information is gained. It is told that the Riders of Rohan were supplied with armor out of Gondor, while the men of Dunland had virtually no mail, relying on their great shields for protection. It's written that they had only a few hauberks, plundered or stolen (At Helm's Deep in The Lord of the Rings it is noted that they have high helms and great shields). The orcs of Isengard made heavy and clumsy mail for their own use. In an initial cavalry advance made by Thedored, the Isengarders had taken up a defensive position, armed with pikes and located behind trenches. There is much written about the use of shield walls when the Rohanners are fighting on foot (shield walls are characteristic of the Classicial/Viking Ages of more recent history). When Theodred makes a shield wall for the defense of the eyot in the middle of the Isen river, a company of "Men or Orc-men", clad in mail (perhaps indicating head-to-foot mail like the orc-chieftain above and not just a hauberk) and armed with great axes, assailed Theodred's shield wall viciously. It was surmised that they had been giving the job of specifically slaying Theodred, since he was heir to the throne of Rohan. They succeeded, though Grimbold and Elfhem made them pay with their lives.