Go do all your research and watch all types of videos like everyone else has said. Then..
Get out a piece of actual paper and MAKE A LIST of things that you need.
Don't just start buying random sh*t. You have to wear these things
together and they have to work together. It'll also help you get an idea of how much this is all going to cost before you start.
Also, don't buy clothes (or shoes) online. You'll end up stuck with a bunch of crap that doesn't fit right that you waited too long and forgot to return.
Jeans:
Go to the actual real-life store and try them on. Buy the slimmest fit that you're comfortable with looks-wise. Slim is no longer super-trendy, it's just
normal now. You don't have to go actual skinny, and probably shouldn't, but if you've been wearing straight or boot cuts then a proper slim fit is going to seem skinny to you.
Plan on spending at least an hour at the store trying on
every possible size that might work. Try all different length/waist combinations and all different cuts. Even if you don't want the skinny look, try them on anyway, in at least 2 sizes. It might make you more comfortable with a "slim" fit after seeing what truly "skinny" actually is.
Don't go too long. They'll bunch up along your leg and you'll look like a grown ass man dressing like a street-walking teenager.
Obviously medium/dark blue is going to be your mainstay, but don't forget black and grey. I love white or cream (ecru) colored jeans too, but they need to be washed every single time you wear them, so that's a pain in the ass. Big attention getter though, if you're comfortable with that.
Don't hesitate to have jeans tailored if necessary. The best jeans I've ever had were tailored. So worth it.
I like Calvin Klein for jeans, personally.
Shoes:
Boots. Wear boots. Chicks dig boots, and so will you.
Chukka boots (desert boots) like these are versatile:
https://www.clarksusa.com/c/Desert-Boot/p/26155484
I have these in black suede and a medium tan/brown. There's almost nothing I can't wear them with, except a suit. I went to the Clarks store the other day and they were practically giving sh*t away trying to draw business.
Or a high boot like this
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www.johnstonmurphy.com
Make sure you get something brown. It immediately sets you apart without being flashy. Medium tan (orange-ish brown) looks nice. Make sure you have a
perfectly matching belt.
Avoid squared toes. That sh*t is over, but they still sell enough of them that you might get tricked into thinking they're cool or in style.
Shirts:
If you have trouble finding shirts that fit then you may actually need to go so far as to have t-shirts tailored. I know it sounds crazy but that's what happens when you're built in the gym. You may not be shaped like the mannequin (or fat guy) that clothes are usually cut for. Find something that fits good in the shoulders/sleeves/length and then have it taken in on the sides to match your taper. This goes for basically any shirt, dress shirts, henleys, whatever. It would be better if you could find something off the rack, but if your shape doesn't allow it then this is what you have to do.
Jackets (blazers) for casual wear:
A medium/light grey and medium blue blazer will get all kinds of use, if you actually want to dress up slightly. Get help with the size from a knowledgeable store associate (or well-dressed friend), and don't skimp on the tailoring. These are great with t-shirts and jeans.
Don't get one that's too long, they can look too formal to go well with jeans.
Avoid fabrics that are too shiny or dressy. That also can look too formal with jeans. Go with something with a heathered or slightly knit texture.
You should be able to get two decent jackets from a privately owned (not corporate) 'suit depot' type of joint, tailoring included, for $300-350. Tailoring will probably be 1/3 of the cost.