I guess it is the one on azurilland.com now (
http://www.azurilland.com/tools/stat-calculator). Maybe it used to be on Marilland (or maybe I used to use another one on there), or maybe I just never paid attention. It's one of the first hits that shows up when I type "Pokemon max stats calc" into Google, so whatever you found probably works. Do be aware that the special stat info may be confusing, as that stat split after generation I. If you know what a Pokemon's base special was in gen I, and what its base special attack and special defense are in gen II and beyond, you can see which stat matches up (so if "special" in gen I has a base value of 80, and "special defense" has a base value of 80 in gen II, but "special attack" has a base value of 100 in gen II, you can look at the "special defense" section of the stat calculator to figure out how good your special stat is). By base stats, if you look at this page on Pidgey, for example (
http://www.serebii.net/pokedex/016.shtml), you can find the "base" stats in a table at the bottom of the page.
For maxing out speed, I mean that you should try to make sure that the speed stat on Pokemon that you're using matches the maximum value for the Pokemon's level given in the max stat calculator. Note that you probably won't be able to narrow down the stat to its exact value until a higher level. The reason why you want to focus on speed in competitive battling, typically, is that it determines who goes first and who goes section, and therefore, it can determine whether you get a chance to attack before getting clobbered by your opponent. One point in speed can frequently make or break a battle, while one point in, say, attack or defense, is unlikely to do so.
Items like protein, etc., add to the "stat experience" of your Pokemon, kind of like EVs in later generations. You can use them to improve your Pokemon's stats, but they'll never change the base "potential" of a Pokemon. You can think of IVs/DVs as being like genes. You could give me all the vitamins and training you want, for example, but I'm still never going to run as fast as an Olympic athlete. Training and stat experience help, but at the end of the day, with maximum training, a Pokemon with a lower speed DV will never beat a Pokemon with a higher speed DV and comparable training (just like I'm not going to win the Olympic 100 m dash even if I train as hard as the athletes who have enough natural talent to compete at the Olympic level).
I agree that you might be better off waiting until Sun and Moon, though. If you really want to train competitively, start learning how to breed and use Super Training on XY or ORAS. I don't know what will happen with the natures. That being said, gender determination actually depends on DVs in the first two generations (that's how they allowed people to move Pokemon from RBY to GSC and assign them genders), with male Pokemon generally having better attack DVs than female ones. (The exact breakdown of how it works depends on the gender ratio. If a Pokemon is 50/50 M/F, the top half of the attack DV spread (8-15) is male, and the bottom half (0-7) is female. If a Pokemon is 7 M / 1 F, males have attack DVs from 2-15, and females have 0-1. Shininess is actually determined by DVs in gen II as well, and as a result of a bunch of factors, you can't have a female shiny starter in gen II [because shinies cannot have attack DVs of 0 or 1], but that's a tangential point that isn't really relevant to your question.)