It's sort of a good and sort of a bad time to start playing; good because the meta is currently undefined as we move into a new format, but bad as a lot of the new standard cards are expensive to get a hold of, and you'll need them to make a deck that works efficiently.
Although we don't know for sure which decks will be the best after rotation, there are some types of deck that we just know will be good based on testing and the information available:
ReshiZard is great, you can find it with the only ball worth using (Cherish Ball) and all the fire support (Giant Hearth, Heat Factory, Fire Crystal, Ninetails for gusting, Firey Flint, Welder) makes this a very strong deck, despite the tools that it loses. The ReshiZard card is probably one of the most expensive single cards out there at the moment, however, along with Dedenne-GX and Jirachi (because you need a playset, and they are £25 each).
If you don't want to play a 3-prize deck, Blacephalong-GX can also make use of the same tools as ReshiZard, but relies also on a Naganadel engine to power it up and can suffer late-game due to the loss of energy to the Lost Zone.
PikaRom is another great Tag-Team deck which stays strong. It has Electromagnetic Radar for search, which can also find Dedenne, and Zeraora is a great addition for free retreat. Tapu Koko Prism Star is busted in any Lightning-type deck with Thunder Mountain, and it has options for damage modification (Electro Power) which most decks don't have. Also, PikaRom cards have dropped in price since they were first released so it's not too expensive to build other than the Dedenne's.
Psychic is another strong archetype as the Malamar engine is just so consistent. Mysterious treasure stays in, so you can pair the engine with the strong Dragon-type pokemon we have currently including Ultra Necrozma-GX and the new Garchomp & Giratina-GX. Baby Giratina works flawlessly in this deck, and with a lot of people investing in Mew & Mew Two-GX (weak to Psychic) it might be a good time to invest in building this deck. Also, if you don't have them already, Malamars will be coming in the Unified Minds Theme Decks which will make them easier to get a hold of.
There's a lot of talk about Dark Box being very good, and some rogue non-GX decks popping up, but these are the 3 archetypes that we know for sure will be strong going into the 2020 Format.