Some good points made above by Rob, the best being that you cant be too rigid in your strategy when you draft. I'm one of those in the camp that would never take a QB early, and wouldn't go into a draft planning on getting a QB in the first 3-4 rounds, but if someone like a Brees fell to me in the second round i would be hard pressed to pass on him. The point is that you should go into the draft with some sort of strategy or direction, but feel confident enough to stray from that plan if you see value that is too good to pass up.
The only real way to know how to properly value guys is to do plenty of mock drafts like Rob suggested. You can find these on a number of different sites and i would recommend doing them on different sites to get a nice variety, different user communities draft differently. Not only will this give you a good idea of value but it will give you a feel for how a draft plays out an when to target different positions. You will quickly see why many people target RBs early, try doing a mock draft where you dont take a RB in the first or second round and see what you end up with.
The key thing to really try and wrap your mind around though is position scarcity, and how those compare to starting requirements in your league. QB for example has 3 guys (Brady, Brees, Rodgers) that are a step above the rest; but then after that there are 10 guys who are all fairly equal. Having someone like Matt Ryan isn't that big of an improvement over someone like a Romo or Russel Wilson so unless you are reaching for those big 3 you can afford to wait and grab a QB later in the draft.
That is why most people say to wait on a QB; there are about 10-12 good QBs, they dont get injured often, and you only need to start 1. Its basically just supply and demand, there are more QBs available than the league requirements demand so their value is diminished because of that. Off the top of my head any team would be happy with Brees, Brady, Rodgers, P Manning, Ryan, Stafford, Cam, Kaepernick, RG3, R Wilson. That's 10 guys right there and doesn't even include the likes of Romo, E Manning, Vick, Big Ben, Flacco, etc.
On the other hand there are maybe 20 solid RBs in the league that you can feel comfortable using on a weekly basis that get a nice volume of touches. Every team starts 2 (with a potential 3rd in a flex in most leagues) meaning that in a 10 team league there is a demand for 25-30 RBs on any given week. Seeing as there are only 20 available you can see that the demand is larger than the supply and that causes their value to be inflated slightly. You add in the fact that RBs are the most widely injured players in the league due to their workload and that raises their value even higher so people try and stock on them to have that safety net in case their guy goes down.
WR is the deepest position in the league, so even though you start 2-3 every week their demand isnt very high. The guys at the very top set themselves apart because of their large number of targets, and then you have about 20 guys below that who are all very startable. Just about every team has 1 viable WR and many teams have 2 so that gives you about 40 guys to fill 30 required spots, plus you get some guys that break out every year so WRs are great to look for on the waiver wire. Try and get a stud early and then load up on potential in the middle rounds.
Tight End is the trickiest position in the game, because there are only 3-4 guys that are really reliable. You can reach for one of them (Gronk, Graham, Gonzalez, Hernandez) but you will probably have to use an early pick on them. If you miss on those guys though there is a group of about 10 guys below them that all have the same boom or bust potential. Its nice to have the safety of a Gronk or Graham but you will probably be using a 1st or 2nd round pick on them. If you dotn get them its probably best to wait until the middle rounds and grab 2 of the second tier of guys and just play the hot hand