Note that these are merely guidelines and pieces of advice, not strictly enforced rules (except that paragraph on mithril, which is a rule from the Imladris Convention). They are intended to help you making your character more canonical and in tune with Middle-earth and Tolkien, as well as your role-play. Straying from some of them may not be a problem, but doing so from many will most likely mean your character fits badly into Middle-earth.

Elves and their background:

There are 3 kindreds of Elves which have different characteristics and historical backgrounds. You should choose one (or more if your character is a mix) that fits you. They are:
Vanyar: Singular Vanya, plural Vanyar. The 1st of the kindreds, and the least numerous. Vanyar normally have hair of fair hue, and the word Vanya does indeed mean ”fair”, and delight mainly in song and poetry. All of this kindred went to Aman and none returned, so the chances of your character being a Vanya are not great. However Vanyarin ancestors are quite likely, and could be an explanation for the many Elves with light-coloured hair.
Noldor: Singular Noldo, plural Noldor. The 2nd of the kindreds, which all went to Aman, though shortly before the First Age a great host of them returned to Middle-earth. The Noldor are thus the only kindred which have Elves in Middle-earth who are Calaquendi. Calaquendi are Elves who are mightier in spirit than others, the Moriquendi, because they saw the light of the Two Trees. Many take advantage of this and proclaim their character such a Noldo Elf-lord of old, though that is not advisable, because it is a difficult role to play well, and others might frown upon it if you are considered young in plaza-terms (as in rank, points, age of membership). Therefore take care before making your character that old. There are plenty of other Noldor born in the 3 Ages of the Sun that you can role-play as. The Noldor, whose name means ”Wisdom”, are the Elves most interested in knowledge and crafts, and have forged and built some of the greatest examples of craftsmanship. Their hair is usually dark, and they tend to be taller and heavier of build than the other kindreds of Elves. In the late Third Age, the Noldor were mainly in Lindon and Imladris, though a small group followed Galadriel to Lothlórien.
Teleri: singular Teler, plural Teleri. The 3rd and most numerous of the 3 kindreds, and also the one with greatest diversity. The Sindarin culture of Doriath was the most refined, though in the late Third Age not many of these lingered. Those who did were mainly in Lindon and a handful in Lothlórien and Mirkwood. Otherwise the culture of Lothlórien was a Silvan one, influenced by the Sindarin one. The Silvan were the most simple culture, and the one mostly attuned to nature and the life in a forest. The Teleri mostly had dark hair except for the royal family of Elwë which mostly had silver hair (the reason for Celeborn’s silver hair). The Teleri are skilled in song and dearly loves water and the song that is found in it.

Special items:

Some items mentioned in the books are rather special, and also used greatly in RPG’s. Here is some brief info on them:
Miruvor: This drink is made in Imladris, perhaps by Elrond himself. It clears the mind and refreshes the body, and is very useful. However, the only ones ever noted to receive miruvor by Elrond where beings like Glorfindel and Gandalf, both being powerful persons, and on important missions. Unless someone is the close friend of Elrond, chances for having miruvor are slim. A substitute such as simple water, sweetened and made more refreshing with herbs, is a more plausible and good substitute.
Lembas: This way-bread is given by Galadriel to those in need of it, and something taught to her by Melian, whom we only once hear about gifting it upon anyone. She gave it to Beleg to aid him in his search for Túrin, and it is mentioned what great honour she bestowed upon Túrin simply by giving it to Beleg. It is thus doubtful if every Elf in Lothlórien has several packs of lembas given to them by Galadriel herself. Common bread or cram, the honey-cakes that for instance the Beornings bake, is a usable substitute.
Mithril: This metal is the most precious of all, because of its light weight and great endurance. It is rare though, especially after the realm of Khazad-dûm (where it was mined) fell to the Balrog and the Orcs. Not many items of this metal would be found in the late Third Age. Therefore it is under the rules of the Imladris Convention banned in an army-RPG for anyone below rank of officer to use items of mithril.

Famous characters:

It is tempting to connect your own character to those mentioned in the books. And quite acceptable when it does not contradict the books or characters. Being the guard of Fingolfin, a scout in the company of Mablung, or a captain serving Gil-galad can add a lot of interest to your bio, and give you some events and timelines to base it upon. However, the problem would come if you contradict the books, or has something that would contradict that character seriously. For instance that you are the close relative of such a famous character, when their family line rules that out, or if it would give you an unfair advantage (for instance being the grandson of Fëanor, claiming it enables you to forge items of far greater worth than the rest of the plaza-members). Or that you are the lost love of Glorfindel. By doing this you create a contradiction to the other players, since they will have a problem whether they should respond to you, or stick to the books/characters. Therefore, having minor relations to famous characters is quite possible, and perhaps even encouraged, since it adds some spice and something your bio can revolve around. But make sure you do not overdo it but keep it realistic and interesting for others, without creating problems for them.