I'd suggest looking at Robert Adams' work. There's little you can't learn from his compositions.

Square format requires a very particular sense of balance, which I feel the rectangular frame does not. You will find that the square can be restrictive in some situations, but this will inform a more specific approach and discipline - which will only make your pictures and style stronger. If a composition doesn't feel right, it probably isn't. Something to mention is converging verticals, which can be that much more jarring with the square. So I'd suggest practicing keeping your camera parallel to your subject for a period until you get a feel for the limits in this regard. As always, this applies especially to architecture and any straight lines in close proximity - arguably with trees too. I always check the spirit level on my tripod from both back and side, unless I'm trying to achieve some kind of effect in perspective - which is rare.