>>170124Learn basic makeup skills. No matter who or what you cosplay, doing good makeup can make any cosplay go from meh to good-tier.
As far as characters, my main tip is it doesn't matter how popular or how obscure the media is, just make sure you are IN LOVE with the character you cosplay. It doesn't matter if it's a character that has one line of dialogue for an entire season or a character you know half the convention will be cosplaying, just LOVE your character. It will make building/buying/wearing your cosplay 1000x more fun.
Craftsmanship-wise, it's always recommended to make a cosplay yourself. Not so much because store bought cosplays are seen as cheap and lazy, but because buying takes most of the fun out of the cosplay. Simple sewing tutorials on youtube are a great place to start. If you are truly clueless about DIY, that's also okay as it's your first cosplay after all. Buying clothes from goodwill and sewing buttons, dying, or ironing on trim is still craftsmanship. You might be impressed by how much you can do with $10 and a lack of a sewing machine.