All these replies and noone capable of answering the question Amarilla posted in the first place.

Closet cosplay is just for dripping one's feet in cosplaying waters and is absolutely not recommended for competitions and conventions. Unless you have no problems to stand in sockings and a t-shirt next to people who poured their hearts, souls and few months of work into their costumes.
Amarilla, the costume on the picture looks like it's been bought ready-to-wear in the shop. If that's a case, avoid going to competitions. Usual requirement for participating there is to have a hand-made costume or at least clothes that have been altered. So if you've made/sewn all or most of the details on your costume, then by all means compete.
As for how to start, I suggest checking with your local anime fan-clubs for the dates of local conventions, or if there's a specific one you'd like to attend there's usually a submition form online where you can apply.