Konnichiwa! 13's definitely not too young to cosplay, I started cosplaying when I was 13 too! ^^
Before you step into the convention venue with your costume, the mirror's your best friend. I'm sure all of us would feel embarrassed, worried and shy first time cosplaying, trust me, I was shivering too, but thanks to my friends' advises, pointers, I feel much better haha.
If you can sew (which I think you're amazing) then you could go fabric shopping at Kamdar, Mid Valley. I've never been there personally, but quite a few of my cosplay friends would go there for fabric shopping.
If not, you could opt for online tailors. Unless you don't really have a budget then you could check out local shop tailors. I've been to a local tailor once and well lets say the quotes were insane, they've made a bleach taichou costume once for approximately 1k. I'm pretty sure that there are quite alot of cosplay tailors on cf forums' market board, and once you've found a tailor that suits your taste I think you'll be fine settling with one tailor.
Personally, I've ordered all my costumes from https://www.facebook.com/nekoworkshop
Here's a few pointers when ordering costumes online.
1. Check what material/fabric your costumes are made with, for example the same character's costume could be made in both satin or cotton material, so if you're not familiar, research more to see what material you think is best for your costume and would look good on you.
2. Provide good references, this would give the tailor better accuracy if the character costume you want is less mainstream. c:
3. Take down measurements properly. Check with your tailor whether they'd prefer exact measurements or do you need to give them 1-2cm difference, since I've got alot of friends receiving their costumes either too big or too small lol.
4. Check for reviews! If you stumble across some forum thread made by a specific tailor, scroll through the board to read reviews from past customers, if its a new tailor, ask for their previous work's reference pics ^^v This is to ensure that you don't get your money cheated or more insurance on whether the product's quality standard is to your liking or not.
Research research and research.
What I do is...either rewatch the entire anime or read the manga all over again like a week or two before the event xD
Then again, mirror-san will be your best friend. haha
Also, if you could, get someone to take photos of you while posing, if you lagi shy, tripod!
Like others had earlier mentioned, taobao. Its also a good place to hunt for costumes/wigs/etc
but like all internet shopping / services got risk lo.
There's a reason why my first cos, also my utmost simplest cosplay actually hit 1k lol.
Hehehe that's all the pointers I could give according from personal experiences in the past. I've only cosplayed like 2-4 characters but I guess those answers are general answers c:
If you still have any other questions, just ask on the forums, a lot out there would be glad to help and share their experiences! v ^^ v
Hope you enjoy cosplaying in the future! c:
Sincerely, a semi-MIA coser haha