The various published rankings do a pretty good job of ranking players within positions (including comparing across age/ability/potential). See FBG rankings, DLF rankings, DLF mocks, etc. The rankings that best match my views on players are, unsurprisingly, my rankings. You should decide before (or during) the draft if you want to try to compete in year 1 or wait and target year 2 and adjust your rankings accordingly (in terms of your willingness to draft old guys vs. developmental guys).
For comparing different positions against each other, a lot depends on your league's rules. In PPR dynasty leagues, WRs have a double edge over RBs - they're aided by PPR, plus they have longer careers. And, for the first 2 rounds of the draft, we happen to be in a time where there are several elite youngish WRs and very few elite youngish RBs. Elite QBs who give you a significant edge on the field have a lot of value if they're young, but right now that's just Rodgers, Luck, and maybe Wilson. Luck is a reasonable choice at #1 overall in most formats (and Rodgers in the first round), but some formats really devalue the QB position (e.g., 10 team PPR where you can start 4+ WRs and only 1 QB). If you can, look at the VBD leaderboard for your league's setup for the 2014 season to get a sense of the relative value of the positions.
Be careful about making trades before you have a sense of player values, especially trades involving draft picks. If you're thinking about trading future rookie draft picks, you can plug in the rookie from this year who is going at that pick to get an estimate of its value.