First thing I would suggest, is to put pen to paper and write down what your game is, how you want it to work, what systems are involved, single player or multiplayer etc. Before you start looking at buying assets etc from the Marketplace, you need to have a good feel about what you want your game to be and how you want it to work.
If you don’t have a good plan to start with, then you’re going to end up buying things you don’t need, keep changing your mind on how you want your game to work, keep trying to add new features etc. Basically you’re going to waste alot of your time on things that are not important or part of your original vision for your game.
Once you have an idea, then start slowly. Take 1 game mechanic for example, and try and get it to work. Not only will this give you experience in UE, but focusing on one thing instead of many will make things easier. Try to learn UE a little before you start buying things, as it is likely you may need to make changes to what you buy to suit your game, and without knowing UE, this can be difficult.
Any changes you want to make to your game are going to be easier to do earlier on rather than late in development, so having a good plan or design document in place at the start will be very helpful.
The forums here and youtube can be very helpful in learning what you need, you just need to know what it is you are trying to do first. Just take your time, have fun and don’t try and learn everything at once, there’s no rush.