I highly recommend charity shops/thrift shops if you're on a tight budget. Additionally, online second-hand stores like ebay and depop are great (though depop can be a rip off sometimes) - some good key words are: neon, emo, kandi/beads, cupcakecult (brand), tutus and of course band shirts of bands u like.
Also you can find really cheap colourful hairdye online if you look for the brand "Crazy Color" so you can dye ur hair multicolour or try out racoon tails. (Or you could buy cheap extensions and dye those)
I'd say if you're just starting out, start with the essentials: hair, basic black or colourful jeans, some colourful shirts, converse or DC's accessories!
Some good accessories are: chokers, studded bracelets, studded belts (you can layer these), wristbands, kandi bracelets, fingerless gloves/fishnet gloves, snapbacks, tiaras, beanies, anything leopard print, neon star necklaces, crazy bands/loom bands, neon bead necklaces, large sunglasses, neon shuttershades, chains/key chains, colourful laces and anything checkerboard, invader zim or pokemon!
DIY TIP: you can easily turn tights/fishnets into shirts so you can have fishnet sleeves (just cut a hole through the crotch and cut the feet off).
Hope this helped :D