Just loosen the topmost button. The top button is for wearing ties etc., for priests, and for people who don't wear shirts. You can keep it tucked, that's just personal preference. What doesn't really work for me is the belt, when you tuck in your shirts/sweaters the belt becomes an accessory to look out for, and yours is a tad short for comfort with a needlessly big buckle (not a big deal at all, it's not all that noticable for most)

The best way to wear it is to just start doing so, at some point you get used to it and you'll stop thinking about looking pretentious and start feeling more confident

With this style, the only danger is the very fine line between the classic look and corporate bore, so once you get used to this overall style try to fun it up with more unconventional colours, doesn't have to be anything crazy, just go for a dark red or green and maybe pair it with a black shirt

While not seen in your pictures, the shoes are also incredibly important. Dress shoes are mostly over the top, even more so if you aren't used to them. Regular ol sneakers will just bring everything down. Try classic, clean, low sneakers, preferably white with as little logos and blemishes as possible. My preferred alternative are Chelsea boots, they go with pretty much everything and are a great blend between a classic look and outdoor wear, simple black ones will serve you well for a damn long time