Decide how much you are willing to spend and what you expect from a skateboard. If you want a decent skateboard for reasonable money, then buy a ready-made skateboard, and if you want something more and are willing to pay for it, you should assemble it yourself.

Choose the type of skateboard: longboard, classic, or straightboard.
Longboards cost $120-$500.
Classics are $80-$400.
Streetboards cost from $50-$150

So, you've chosen to buy a ready-made skateboard. The first thing you need to do is find a good store. While buying online may come out a little cheaper, by buying from a store, you're not only supporting the store, but the entire local skate community as well. Plus, over time, you'll get a discount. While a pre-made skateboard is not as good as a self-assembled skateboard, it also costs a lot less.
A regular skateboard costs in the $50-$200 range.
A self-assembled board costs between $90-$500.

How much are you willing to shell out for a deck? If you want to save money, you can buy a clean deck (no pictures or lettering). They cost a lot less, although they are just as good as branded boards in anything else. However, if you have a dream of having a skateboard like your favorite skateboarder and have the money to do so, buy one.

Buy a quality deck. Think twice before buying a deck from local stores - their quality leaves a lot to be desired. If you're buying a pre-made skateboard, then order it online or go to your local skate shop and ask the salesperson to help you choose. If you want to just skate, the deck is suitable width 7.5-8.0 inches (18.5-20 cm), for tricks suit more boards from 8 inches and wider.
Another important consideration is your height and the style in which you are going to skate. Most technical skaters (like Rodney Mullen, who uses a lot of flips) prefer 7.5 and 7.75 inch (18.5-19.5 cm.) decks regardless of height. You can do spins faster on these non-widespread boards. On the other hand, extreme skaters (Jamie Thomson is a classic example) prefer wider boards. Their advantages include better stability in the air and on landings. If you're a teenager, a board 7.66 wide or smaller might be the best option.

Tracks are the second most important part of the skateboard. When buying them, pay attention to the following characteristics.
The length should not exceed 10 inches (20 cm).
The width should correspond to the deck. For example, 7.5 tracks will fit a 7.5-inch wide deck.

Ignore the design. Good looks are not a sign of good quality.
Choose wheels depending on the skateboard. For a street deck, large and wide wheels are fine.
Longboards fit big and soft wheels.
Streetboards fit small and hard wheels.
Try to choose wheels of the same brand as the deck. Otherwise people might think you're a poser. If you have a deck made by Almosts, DGKs and so on, then wheels from any brand will do.

Find the right bearings. Bearings are available in numbers from 1 to 9 (1, 3, 5, 7 and 9). The higher the number, the higher the quality of the bearing grind, and the better such bearings spin and last longer. The best choice for your first skateboard would be number 5 or 7.If you start with number 1 and then change them, it will be difficult to get used to new ones that are much faster in the future.

Before you buy a deck, take it off the shelf and stand on it. Make sure that the length and width fit you. Do this with the shoes you intend to ride on.
Don't buy a board that just looks cool, much more important is how it will feel under your feet.
Before buying, it is best to go to the nearest skate shop and ask for advice. Buyers and sellers can usually be of great help, picking up a board that is right for you. But remember that, as they say, as many people, as many opinions. Some people like Plan B boards and some people like DGK boards. So it is better to try out the board yourself and make sure it suits you.
Sometimes in skate shops you can find clean, good quality boards that will last a long time and come out cheaper than the branded ones.
Of the wheel brands, check out Spitfire, Ricta, Bones'" and Autobahn.
If you're tired of the problems of peeling skins, buy Grip gum. It costs about $14.
If you have a clean board, you can paint it yourself. All you will need is a stencil and paint.
You don't have to have very high quality, smooth bearings for a skateboard. You may never need high numbers. Some companies don't even number them. However, what you pay for is what you get, and quality is worth the money.
Grind King, Independent, Krux, Thunder and Silvers are probably the best brands among bearings.