Brand doesn't matter. Wear whatever clothes is reasonable (don't workout naked!), and comfortable for you. If you want shorts with zippered pockets, then look for that. Typically I'll wear a tank top/t-shirt, shorts and sneakers.
If you're just starting out, what's more important is that you show up at the gym consistently. You want to go regularly so that you body can adapt and get stronger over time. This means going when your motivation is low, going when you're busy, going when you're feeling lazy, and going once again after you stopped going for a bit.