Like Onimiv said ..... this is too big to start ....
BUT.
Write it all out on paper .... everything .....
For every menu or dialog or setting .... Draw a screen shot (What does your character generator look like) and so on.
People here have released countless games... and every game started as a drawing on a napkin.
Currently I am looking for a project to do.... I have 2 that I am supposed to do, but I hate drawing.
I did not realize how much I hated it until I decided to draw and animate all my own creatures .... Made 15% of the creatures
before loosing motivation and moving on to experimenting with neural networks (some serious math there, which I love).
Unity ... My chief complaint with Unity is it is pretty heavy duty. Many games created with Unity and designed to play in a browser
become so huge they become laggy or have other issues. Some dev teams know this and can work around it using special techniques.
If starting out , brand new, look for a free javascript engine. There are many available.
here is one (I don't use this one, but it looks cool).
http://www.tululoo.com/That one does not require code experience, just some knowledge how javascript works.
Good luck on your project....
PS
Start small.
Break your game up into many many mini-games...
1 - simple duel game... pick 2 creatures and they fight. simple concept
but it lets you experiment with each character for you game
2 - map editor or selector (seems big) but can be pretty simple
You just need to make a few maps, even notepad works for this
This lets you experiment with loading and saving data .....
3 - different dialog boxes for all your settings ...
naturally, you will need to save and load these settings
4 - character generator or map generator ....
make these stand-alone until you have everything working alone...
5 - after you complete all this .....
now try putting it all-together.