Band t-shirts are cool. You just have to make sure you're wearing a cool band's t-shirt. I can not stress this enough. You can't just slap on a Lynrd Skynrd t-shirt and be emo. You might be able to get away with an intentionally ironic shirt - something like Cinderella or Winger, but you better have the rest of your emo fashion in order before even trying this.
Without emo kids and emo fashion, the hoodie business would collapse. In my house, every time I come home with a new hoodie [and I have a bit of a thing for them], my boyfriend jokes about how "desperately" I needed a new one. And then gets mad at me for stealing his. Hoodies are just a staple of emo fashion, and a decent collection of them is essential to any emo wardrobe.
That said, it's important to have an emo hoodie. Because other people do wear them, you can't show up in a sports team or a college hoodie and be emo. You'll look like a dork or a poser and not remotely in touch with emo fashion. Band hoodies? Good. Emo-approved clothing company hoodies? Good. Skulls, stars, cute designs on the hoodies? Good. Plain hoodies? Acceptable. It's really easy... don't wear anything that would lead someone to mistake you for a frat boy or frat girl and you're probably good to go.
Tight, not baggy. Boys used to be all but obliged to wear girl's jeans, but that standard has been significantly relaxed. Feel free to wear boy jeans, if you're a boy [but not if you're a girl!], just make sure they fit. You don't want them hanging off your ass or ten sizes too big! You're not a thug or a skater circa 1994.
Jackets aren't very emo; get some more hoodies. Wear a lighter hoodie under a heavier one, if you're cold. Gas station jackets are also okay, but not as cool as they used to be. Nonetheless, you won't be shunned like you have the clap at an orgy if you show up wearing one.
