Recently had a similar experience/question. Here are my Tips (assuming you know very little about the current state of PTCG):
1. TOP ADVICE: Make an account on Pokemon TCG Online.
Its a great way to play the game with others remotely, build decks for cheaper, and play when you want where you want. If you like the TCG and want to play or practice, you're going to end up doing most of that on PTCGO instead of the once-a-week LGS Play! league you might go to. Best part about PTCGO is that once you have a copy of a card, you can use it in as many decks as you want, so once you have a decent amount of cards you can really build any deck you want at any time.
2. The easiest way to build decks is to buy singles!
There are many websites that sell single specific cards, the biggest one in the USA is TCGplayer.com, although I believe CardCavern might have a bigger UK market. Do a little digging into the site that works for your location. If you're looking to build a deck, its always going to be cheaper to buy the singles you're missing then to go hunting through booster boxes.
3. A first buy should be the most recent ETB (Elite Trainer Box).
This will get you a lot of the needed game pieces like dice, counters, sleeves, etc, + a box to store cards/decks in. Usually $40-$50USD depending on the set and where you buy it from.
4. Find an IRL league and/or online forum to trade physical pokemon cards.
If you do have the money to buy booster boxes, there are plenty of forums, discords, subreddits (particularly r/pkmntcgtrades
) where you can trade or sell the better cards for cards that you want for your decks. I will often buy a booster box per set, and coast on the value pulls to build my decks for the next 3-4 months before the next set.
+ Additional note: USE THE CODES YOU PULL FROM PACKS! Those codes get you 1 pack in PTCGO, and can be used as "currency" or opened for online cards.
5. Lastly for deck building, watch fun content creators.
A personal favorite of mine is Little Dark Fury and the Shuffle Squad. You can find lots of deck ideas, fun TCG content, and general competitive information for getting into the game.
With that being said, the format is about to shift heavily in early 2023 and the new Lost Origins set is going to change up the top decks, but I'll still give a good meta/low-cost/fun deck idea that's still viable: The Regigigas Deck
and/or Radiant Charizard
Hope this Helps!