People aren't going to look at you weird for being in the men's section, unless you are the one acting like you shouldn't be there. If you're looking around paranoid like, yeah people will notice. If you just walk in like you belong, people won't pay attention, especially at big stores. If you don't act like its anything weird, neither will they.
If you are still presenting female and not out, make up a story about why you're there first. "I'm shopping for my boyfriend/brother/cousin for his birthday/Christmas/whatever." Already having a back up story will make you feel more confident, but honestly, I've never had anyone ask.
As for what to wear, I started with a pullover and a button up shirt that I wear over my more feminine clothes. The only reaction I got from my family, who I wasn't out to, was "that's a nice shirt." It made me feel more masculine without broadcasting "I'm trans!"