>>713233151
To begin, you should buy the staple packs that cost 700 gems and the season pass (another 700 gems).
Now talking about decks specifically, I'll give you the non-retarded answer. Buy 3 Dragon Maid structure decks (this should run you 1500 gems and carry you all the way to gold) and HOARD every other gem you get from solo mode and rank. When you're a new player you get an insane amount of gems. Currently, we don't have good selection packs available, but a few really busted decks are on the horizon.
If you wanna improve the deck put every staple card you bought, look up meta versions of your deck and craft some cards that seem useful (mainly N, R and SRs). Also, if you have friends who are on the verge of installing the game, there is a campaign rn where you can get a Branded structure deck if they use your referral code, so do that (you can even combine Dragon Maids and Branded because both are fusion dragon decks).
Ask more questions in these threads if you have any (it's unironically the best place to get MD knowledge)