Wall of text incoming!
When you think of 40k, remember this quote: ?it?s like crack, but more expensive?. I cannot tell you how true this is. To help with this there are some good website with better prices than the Games Workshop website to check out. If you PM me I'll send you the links and my two cents on them, I don't want to break the advertising rule or I'd post them now >_>
Back on topic however, really you just want to get the army that speaks to you the most. Look at them, read through their ?fluff? (background, etc.) and if you find yourself saying OMGAWESOME then seriously consider buying them. However, do your homework first. Watch some battle reports, do research, check out wargaming forums and army lists to get a feel for your army. This is just to avoid jumping in and regretting choices. If you go with Spess Mahreens (Space Marines), I highly recommend the Black Reach set as previous posters have pointed out. A few other people have given brief synopsis on the four armies you chose, but I?ll go into a little more detail on a couple from what I know.
I have four armies, Black Templars, which are Space Marines that prefer to hit people which chainswords instead of shoot them, Eldar, Tau, and Orks.
Space Marines: Your key with these guys is the fact that they are nigh indestructible. They have a good toughness value which means most weapons will have issues actually wounding them, and they also have a 3+ armor save which means that most of the time you will only need to roll a 3 or higher on your dice to deflect most of the shots they will take. This makes them incredibly survivable assuming the dice gods don?t hate you. However, they are very expensive per model on the points scale, and thus your army will normally be outnumbered. These guys normally win simply out of sheer survivability. Normal Space Marine armies do have access to Assault Marines and other units which have close combat capabilities, as well as some very mean vehicles. If you are looking for a well balanced army that is hard to knock over and is pretty forgiving on your wallet, these are your guys.
Eldar: Elves in space. These guys are incredibly specialized. Each squad is good for one thing, and not much else. They are not super survivable; as a previous poster said ?paper tigers? are a good way to summarize them. However, they have some awesome firepower and a unit for almost every situation. They have two main close combat squads, Striking Scorpions which are good for ?squishies? (Imperial Guard, Orks, Tyranids, etc) or Howling Banshees which are better for heavily armored foes (Space Marines, etc). They have a boatload of shooty squads, I won?t get into those, but their main ones are fairly short ranged, but the right ones for the right job will vaporize what you need it to. Their armored choices are pretty awesome and all of them are extremely fast and ignore most terrain (skimmers), but they are expensive compared to others. However, the incredibly specialized nature of the Eldar makes them hard to play, and you?ll end up needed a pretty decent amount of models. Also, if you fudge up a few times, kiss yourself goodbye. Expect to lose a lot of games until you learn how to use them. Not as cost effective as the Space Marines.
Tau: ?Yeah, we suck in close combat. Too bad you?ll never get there.? Now, this is true, they suck in close combat as everyone says. A guardsman will have a pretty good chance of whooping on a Tau Firewarrior in CC, and guardsmen are meatshields with guns. But God help you if you stay in their line of sight for more than five seconds. I am currently building my Tau army, and I have not gotten to play them much, so I?ll just state what I think I know. I do know they have some of the most effective anti tank weapons in the game, very very high strength and high armor piercing. Their basic infantry has a really good range and their weapon has a really good strength value, which means more killy. However their leadership value is not so awesome, and for that reason they often run away if they take a goodly number of casualties. They do have a CC group, Kroot, which aren?t too bad at their job but from what I have observed, more often than not they are just cannon fodder to allow you another shot at your enemies. They also have stealth suits, which can be a pain for your enemies to kill off because of that whole ?invisible suit? thing. Their main Elite squad is the Crisis Suit, which is a basically a mech. They are pretty versatile, and are very mobile. The whole Tau army itself is pretty mobile really, and they seem to have a decent survivability rate given their armor save (so long as they are being shot at, and not beat down).
Imperial Guard: I don?t play them, but I play against them a lot. These guys are known as ?The Hammer of the Emperor? for a reason, they beat you into submission through numbers. They Imperial Guard way is pretty much do stuff in waves of bodies and you will eventually win. Their basic troop has a less then awesome armor save, not so great toughness, and a weapon comparable to an overpowered flashlight (the Lasgun, known as the ?Emperor?s flashlight? by us geeks). However, with the last codex especially, these guys are incredibly versatile and are not to be underestimated. As soon as you do underestimate them they will sting you to death with their flashlight and bayonet you in the face just for fun. They have a considerable armored selection including the Leman Russ tank, which I consider one of the meanest tanks in the game. Also, there?s not much that?s cooler than a whole bunch of regular dudes with underpowered, cheap weaponry and a flak vest taking on bioengineered killing machines and daemons. It?s just kinda heroic.
Though I hate to suggest you turn to the heretical forces of Chaos, you may also want to consider Chaos Space Marines. They have some other interesting choices and are a bit different than just regular Space Marines. I don?t know much about them however.
Anyway, that?s my two cents. Glad you?re getting into the hobby!