How to make a good deck?

If you are stuck in gold league for a year, it's not only the deck. Maybe it is a good idea to build one deck and play it a lot, so you can get insight on how to make the best out of it, instead of trying alot of different builds.

For beginners, I guess gravestone decks (impfinity or super brains) are the easiest to play and strongest.

On how to build a good deck, well, thats a tough question. You have to decide first, how your deck wants to win. If you want to win with big cards, your deck needs cheap cards to control the game. If you want to out-pace your oponent, you need minions with high attack and some quick removal, to clear their path.

I dont know to what cards you have access, but even with just the base set, you can build powerful decks.

If you have some sparks to spend, get 4 Pineclones as they will really push your deck forward. you can combine them with shroom for two and all the good mushrooms you have (punish, muscle) Add in some big finishers and 4 berry blast and you already have a really good deck.

For zombies its similar. you can play all the basic gravestones and add some mixed up gravediggers and youll be fine. 

Its also important to not play cards that counter each other. for example smoke bomb is good if you have empty lanes, so its worse when paired with a lot of tough gravestone zombies.

Last but not least, you need to learn what cards counter your strategy. You can play around chickening and weedspray, but if you always lose to toxic waste imp, you probably should add a 2 power amphibious to your deck. just play some games with your deck and see what cards dont impact the game, which cards are always the last in your hand and what cards your oponent plays to trash you. from all that, you learn how to improve your deck.

its also important to look at the curve (what all the cards cost) of your deck. it really depends on your strategy, but overall, i play around 20 cards that cost 3 or less and 20 that cost 4 or more. you need at least 6 cards that cost 1, otherwise you will get overrun frequently. in my experience, playing more than 4 cards that cost more than 6 is problematic. 

i hope this helped you and wish you a nice day.