Me and my friend are building a "simple" trading card game - in the beginning we would have 20 cards and two resources, mana and health. This, along with a working multi-player is the minimum viable programm. The programming and graphics are not a problem - not of course literally, but since we're in the design state, we have other questions before coming to this two topics. Currently we have 10 cards, some basic mechanics and a short draft of ideas/comments/etc that I call the Design document. What I'm unsure now is how to continue developing the game, so I guess what I need are resources for game theory/game development so we at least don't reinvent everything.
Edit: To answer the questions below and keep them more visible:
This is the game in it's current form (made for iPad but works on Desktop computer if you use A S D J K L): http://app.planetofsnail.com/game.html As you can see, this is not a trading card game (or anything more than the bare skeleton of the idea :) The reason I called it a TCG is because of the way you play the game: you tap combinations to create magic symbols that translate into spells. When you break it down, it's like playing Magic the gathering, but instead of collecting and tapping lands, you make some hand gestures :) You have spells and monsters and two mana cast cost translates into two cast combinations (not exactly, but this is good enough without going into unnecessary details).
So to sum it up, from M:TG it uses literally spell names, mana costs and the fact two players play the game. The actual mechanic is different (you don't collect lands, every turn you have a fixed amount of mana that replenishes etc.)
BerndBrots' link to a game balancing article and actually the whole series is what I'm looking for - how to design a game (preferably TCG since TCG is the closes to what we're doing).