First, know your measurements. Research tailors/alteration places that have good reviews in your area and get to know them. Most places you can go in and just ask for your body measurements like inseam, waist, sleeve, shoulder, etc. Just getting your measurements should be cheap (free sometimes if you go to a mom and pop type place). Knowing these are vital for online shopping, which is where you can get the best quality for good prices many times. Knowing your measurements also allows you to buy clothes that truly fit, which is the most important thing. Fancy clothes still look bad if they're too tight or baggy.
As for what clothes to buy, the best place to start would be with the basics. It sounds obvious, but there are certain staples that are timeless and go with a lot of things and different styles:
1. Dark indigo jeans
2. Khaki and Navy colored chinos (no pleats)
3. Khaki and Navy colored shorts (no cargo)
3. Solid color tshirts and polos (no brands/graphics, you want versatile to build a foundation)
4. White canvas sneakers and brown or black boots of some kind
From there you can use your existing clothes and just build your own style one piece of clothing at a time. A lot of what you wear depends on the climate of where you live. As someone who lives in Florida, I have more linen and seersucker shirts than someone who lives in a colder and less humid climate. Check out various style forums to learn more once you get going, and don't feel like what you read are hard and fast rules. You can break the rules, but you do have to learn them before you are able to do that effectively. Have fun!