Wear the underwear that fits. You will undoubtedly be wearing jeans or something like Carhartt duck work pants, so make sure that your underwear fits and is comfortable. You *will* be moving around while cutting dovetails. And you WILL be squatting and crawling around while doing the plumbing.
As for the socks, there are specific socks for steel toed boots. I've seen them from a number of brands, but Dickies and Carhartt are commonly available in the US. Basically the only thing special is that the toes are reinforced and extra-thick.
I'm not certain what brands are available where you live, but surely something comparable is available at work-wear shops.