As a Prot specced Paladin, you will likely never die (unless you aggro too many casters at one time), but you will find that you do not do a tremendous amount of damage, especially at the lower levels.
The only reason to level as Prot is if you plan to do a ton of instances on the way up from 1-60 and want to tank (you can also tank in Ret spec in any dungeon prior to outlands if you use a 1-hander and a shield, so even doing a lot of instances, Prot is not really necessary).
The vast majority of Paladins choose to level as Retribution because the speed with which you kill things is just so much faster, and by the early 20s you can have 2 points in Pursuit of Justice which allows you to travel (walking or riding) 15% faster. Between 20 and 30 there are a ton of quests that have you literally running all over the place, and PoJ making that take 15% less time really adds up as you level.
There is nothing "wrong" with leveling as Prot spec. It is perfectly feasible, it just takes longer than leveling as Ret. If you chose to level as Prot, simply expect to run a lot of instances and/or have it take a bit longer to level.
In the end it is your choice, and there is really no "wrong" choice (although VERY few people level a Paladin in Holy spec). Most people simply level Ret because it really is faster that way. If you do plan on tanking and you haven't done it before, respec Prot at 60 (when you will have the vast majority of your key tanking abilities) and tank dungeons in Outlands/Northrend from 60-80. Before that, you can throw on a 1-hander and shield and put Righteous Fury up and tank in Ret Spec in any old-world dungeons, or just keep your big slow 2-hander equipped and go as DPS.