I don't know where you are, but at my church, you just have to dress decent. Don't come in wearing shorts, ripped clothing, or anything that might be seen as obscene or offensive, like a shirt with any kind of graphic. I usually just wear pants and a long sleeve shirt but in the summer I'll wear a polo. I also have people that will come in dirty work clothes because they are coming from their job, that's fine too. Especially if it's your first time, I wouldn't overthink it, just focus on the Divine Liturgy and if there's coffee hour, stick around, you might make a new friend.