If you plan to make a living out of this, you should start by having monetisation included from the very beginning.
Even further, start with what do you think would sell and build the game around that.
So many people are against F2P (Free 2 Play) and at the same time they want to make a living with their company making games. There's a bit of a contradiction, isn't there?
There're many ways to do it. You yourself should consider it as a mean of help to the player, not exploit. I always refer to how it is done in Candy Crush Saga. It helps you and it's not pushing you to pay for something meaningless as soon as possible.
You should realise you're not making a game, but a product. You need to think for who you are going to sell it. And what do you sell them.
You can use similar things as in your local grocery store
As funny as it is, we take off some time to play a game and to relax.
But then we don't advance in the game.
So we pay to advance.
Time is what people pay for.
There are other things, like extra moves, but again you save time and are able to advance to next level when you use money.
Love, trouble, help - make your player's love the game, then put in some sort of trouble from which you offer help. The trouble can be like