You should be able to play the game without getting too confused, as Shadow of Mordor is relatively "isolated" from the rest of the Lord of the Rings as far as the story is concerned.
One part that might be confusing to people who don't know anything about LotR revolves around the Rings of Power, specifically the One Ring.
The Rings of Power were rings which gave their bearer tremendous power. They were given to Human, Dwarven and Elven kings by Sauron, the main antagonist of the Lord of the Rings. Later, Sauron forged "the One Ring to rule them all" in secret and used it to gain control over the bearers of the Rings of Power (except for the Elves).
Shadow of Mordor explains how the Rings of Power and the One Ring were crafted, despite them being mostly irrelevant to the rest of the game's story. If you don't know what those rings are, you might wonder why they are even mentioned.
Then, there's Gollum (also known as Sméagol). His involvement in the game's story — which is related to the One Ring — can be really confusing if you don't know his story.
After Sauron was defeated, his Ring somehow ended up in Gollum's hands. Over the centuries the One Ring made him lose his mind and he eventually lost his "prrrecioussss" to a Hobbit named Bilbo Baggins. After having possessed the One Ring for so long, Gollum is obsessed with it and wants it back at all costs.
Finally, there are the Uruks. Luckily, you won't need any background story to know that Uruks are this game's monsters which you'll be killing a lot.
One more thing worth mentioning is the fact that Shadow of Mordor holds a lot of information about the lore of Tolkien's universe.
The White Skull can be considered the generic one because it just indicates that the Uruk in question is a Captain
The Blue Skull is a Branded Uruk Captain, branded by you ofcourse.
In Battle you can have Yellow Skulls and Green Skulls. Yellow is when you are detected and that Uruk captain is investigating and Green is when he can be finished off in battle.
There is a variety of all the Skulls with a Red indicator, as you can see here:
In the picture you can see a White Skull with the indicator but it can be any type of skull.
Best Answer
Simple answer
You can buy Middle-earth: Shadow of Mordor straight away.
More detailed answer
There really is no reason to play the games in a specific order, they have very little in common. storywise, they all are pretty much completely independent from each other.
What I know about the games in this list: Fellowship, Two Towers and Return of the king (2002-2003) are third person action games, which follow the movies fairly closely. Battle for Middle-earth 1 and 2 are great RTS games, whereas the first is close to the events of the books/movies, while the second looks at other parts of tolkiens mythology. Both feature a mode, where you can conquer the whole of middle earth (similar to the total war series). Despite being a series, the games really don't have anything to do with each other in terms of the story they tell. However as EAs license for LotR games expired it may be very hard to get a hold of those games.
War of the ring(2003) is not based on the movies, but the general tolkien license, it's a new story, but I haven't played the game.
War in the north(2011) is an action oriented-rpg featuring a storyline not covered in the main books or books. It's nice for a fan, but the game is nothing spectacular.
Guardians of Middle-Earth(2012) is a moba(think league of legends or dota), I haven't played it, so I can't tell anything more about it.
Shadow of Mordor(2014) is a 3rd person open world action game in the vein of assassins creed. The story isn't covered in the books or movies. It's a very good game with a great feature in the nemesis system, but like AC-games it suffers from being incredibly easy.
The big one is Lord of the rings online. It features a massive game world, a long sotryline quest series, which touch both new ground and elements from the main book series. It's a wealth of content to explore, but of course it is an MMORPG with all its drawbacks and trappings.
I can't say anything about other titles in the list.