The one criterion that actually matters
Buy a deck with illustrated Minor Arcana. This single factor affects your learning more than artwork, size, price, or theme combined. In an illustrated deck the Five of Cups shows a cloaked figure and spilled cups, and you can read it by describing what is happening before you know what it means. In an unillustrated deck the same card shows five cups arranged on a plain background, and you cannot read it at all until you have learned a system of number and suit correspondences.
Both approaches are legitimate and only one of them is a reasonable place to start. The Tarot de Marseille is a serious tradition with a serious method; it is a poor first deck for the same reason a manual gearbox is a poor first car.
How to tell whether a deck will match what you read online
Nearly every English-language card meaning (in books, on apps, on this site) describes Rider-Waite-Smith imagery. A deck in that family will agree with your learning material; one outside it will quietly contradict it, which is confusing in a way beginners usually blame on themselves.
The fastest test is the Three of Swords. If it shows three swords through a heart, you are in the RWS family and the meanings will fit. Check the Eight of Cups too: a figure walking away from stacked cups under a moon is the other reliable marker. Clone decks such as Universal Waite, Radiant Rider-Waite, and the Smith-Waite Centennial edition are the same 78 scenes recoloured, so every meaning transfers exactly.
The physical things nobody mentions until you own one
Standard tarot cards are noticeably larger than playing cards, and if your hands are small a full-size deck is genuinely hard to shuffle. Several popular decks ship in a pocket or mini edition, which is worth considering before you buy the one everyone recommends.
Card stock matters more than it sounds. A thin, flexible stock shuffles beautifully and creases within a year of daily use; a heavy laminated stock lasts and fans badly out of the box. Matte finishes resist fingerprints and scuff; gloss finishes stay clean-looking and stick together in humid weather. None of these ruin a deck, and all of them are things you notice every single day.
The gift myth
You will read repeatedly that a tarot deck must be given to you rather than bought. There is no historical basis for this. It appears to be a twentieth-century piece of folklore that spread because it sounds like the kind of thing that ought to be true. It has also kept a lot of people from starting.
Buy your own deck. Choose the imagery you actually want to look at every day, since you will be looking at these 78 pictures for years, and the aesthetic response you have to them is doing real work in how you read.
A reasonable first purchase
If you want the shortest defensible answer: a Rider-Waite-Smith deck or one of its clones, in whatever colouring appeals to you, in a size your hands can shuffle. It is the deck almost all learning material is written against, and starting there means the deck and the book agree.
Buy the second deck when you can articulate what the first one does not do for you. That is a much better reason than novelty, and it usually arrives around six months in.