Wear a belt (leather/reversible) with pants, also the belt shouldn't be holding up your pants but instead just make them look better so buy pants that fit. (Slim or Straight/slim fit usually look the best, never boot cut)
Your ankles shouldn't be visible under your pants when you sit down (longer socks)
Give some consideration to what looks good on you. That means colors, also body shape etc.
Beyond that you get more into specifics.
I just recently went through an entire wardrobe / style update, so I know exactly how overwhelming it all is. I ran across Aaron Marino of Alpha M on YouTube, and watched all 100 of his free videos. Aaron is the man, and he definitely helped me move forward with confidence to update my entire wardrobe.
Some of the big takeaways I've learned:
Fit is everything, its important to learn how to pick the right fitting clothes, too tight or too baggy kills the look. Make a promise to dress well every time you leave the house. I used to only get dressed on the weekends or for special events. I traded all my college / sports T shirts, for stylish casual V-necks. A nice dark wash fitted pair of jeans was the best investment I made so far. That's all I can think for now, but check out Aaron Marino, He's the Man!