I would like to go to our local anime convention AnimeNEXT (in America) as Princess Peach. Now I know your wondering.. what's the question? Well I was thinking of making a gothic/dark version of it being all black, but still having the token blue eyes and blond hair, along with the crown. I am going to be carrying around a toadstool purse so people would sort of get a clue of who i am. And i'm going to incorporate the gem.

But I don't know if it would be better to buy an already made costume and die it black or just find pieces and put it together? I really need help with this because I want it to look nice and creative. PLEASE HELP!

I'd recommend not buying a cosplay and then dying it - the majority of cheaper costumes are made out of synthetic material, and do not dye easily or don't dye correctly.

As a quick example, you could use something like this http://www.ebay.com/itm/VINTAGE-TADASHI-BLACK-TAFETTA-SATIN-BALL-GOWN-STRONG-SHOULDER-TULLE-UNDERSLIP-/300844735511?pt=US_CSA_WC_Dresses&hash=item460bbe5c17

as a base. Then add the bustle and such to it.

Also. I want to make it a bit shorter than regular Princess Peach length. Since it is going to be in the summer, and I don't want a large long dress. So i was thinking maybe a shorter dress, or a skirt? But I don't want to take too much away from Princess Peach but I also want to make it my own. I don't want to be abad cosplayer.

You're doing you own design of Princess Peach. As long as, to the end of it, you still have iconic parts of Peach in it, you ought to be fine ;3 I'd recommend sticking some pink in there (like in the underskirt, or something, so it's all black but then you get a small flash of pink if you lift your skirt to ascend/descend stairs - that would look nice). Be aware that, no matter what, there will always be people who'll be like "god! You're a bad cosplayer cause you did something creative with a character" but yeah, keep in some key aspects and it'll look nice :3 Like keeping that ballgown shape - you could tatter the bottom to shorten it, or bring it up at the front or something ;p

BLOCK OF TEXT! sorry, I'm trying to make a pattern piece at the same time.

I'm just so scared, because I am new to cosplaying & this is my FIRST anime convention EVER. So i know I'm not going to do it perfect the first time around.

Since it's three days the first day i'm going as:

http://www.cosplaymagic.com/blcisaco.html

because my friends & I are doing Bleach theme.

Then the next day I was to do Peach, but I want it to be like this...
but I don't know if it's too away from the idea of Peach.

http://www.ezcosplay.com/super-mario-bros-princess-peach-adult-sexy-costume.html

i'm trying to look nice, not too tacky. ^_^