How to Dress Classy as a Man

A plain white t-shirt that's clean, well-fitted and in good condition looks better than an expensive shirt that's creased, dingy or has loose threads hanging off. Fabric bobbles are those little balls of fluff you get on jumpers and sweatshirts after a few wears and washes. They make clothes look worn out even when they're not that old. A fabric shaver costs next to nothing and takes two minutes to use. That can bring a sweater back to life fast.
Yellowing around collars and underarms on white shirts is never looks good. Some shirts can be treated and recovered, but if they can't, they need to go. 

Shoes are one of the first things people notice, even when they're not consciously looking. A good outfit paired with dirty, scuffed or down-at-the-heel shoes looks like the shoes belong to the real you and everything above the ankle is borrowed.
Clean your shoes regularly. Not just a quick wipe - actually clean them. Get a kit suited to the material, whether that's leather, suede or canvas, and use it as directed. Leather shoes benefit from polish and conditioning. Suede needs a brush and a protector spray. White trainers need a proper clean, don't just leave it to the washing machine to sort out.

A well-dressed man who hasn't looked after his grooming is like a tidy house with an unmade bed - you immediately focus on the one thing that wasn't done and wonder why it wasn't done.. That the same way being well-dressed and well-groomed always go together.
If you want to go deeper into the basics, this guide on everyday grooming habits breaks it down properly.

A good quality watch, a leather or beaded bracelet, a belt that matches your shoes, and a clean pair of glasses or sunglasses if needed. That's usually enough. You don’t need to add layers - a chain necklace, a second bracelet, a cap, multiple rings, a statement belt buckle turns into noise. Nothing stands out because everything is competing.
Pick one thing you want to wear and let it do its job. If you're wearing a watch you like, let the watch be the thing. If you've got a jacket you enjoy, let the jacket be the thing.

This is covered properly elsewhere, so this isn't the place to go deep on it. But it's worth saying once: everything you just read above, only works if the clothes you're wearing actually fit you. A perfectly conditioned, beautifully maintained shirt that's two sizes too big still looks like a perfectly conditioned, beautifully maintained shirt that's two sizes too big.
Fit is the foundation. The details and habits covered in this post are what you build on top of it.