Yeah, start off with the breedable legendaries. Search for the Monster Legends Wiki, it's the essential resource for this game. Focus on getting your gold production up. This will allow you do get your food production up and get all needed buildings and boosts. Social Point has nerfed a lot of big gold producing monsters, but the best gold 'farms' are legendaries, without question.
As events happen (and they are literally ALWAYS happening) you'll start earning new legendaries. As you progress, you'll be able to do better in the challenges (like feeding monsters, collecting gold, crafting/buying runes) which will net you gems. And you'll be able to blast through dungeons, which will give you even more gems.
As you have gems, you can purchase monsters when they're on sale (don't buy ANY if they cost more than 276 gems). They often offer 3-packs where you get 3 legendary monsters for an even better discounted price. I've often bought those. As a general rule, don't use gems to speed up buildings/breedings/hatchings.
At some point, you'll have to figure out what your motivation in the game is. For me, I like collecting monsters. I know I never will, but I want to "catch 'em all." Some players, however, want to be the very best (like no one ever was). Those players spend gems to buy duplicate monsters (to rank them up) and high level runes (necessary to start focusing on at some point).
Lastly, this is a game that rewards you for constantly opening it up and playing. And OVER TIME, you will be doing quite well in it. Have fun!