Here is everything you need to know.
Just in case that is way too long for you, here are the cliff notes:
In the garage you compete against a predefined "game that does not exist" so once you have a good game engine you'll get more money.
About when the Gameling comes out, that is around the time your "smash hit game" will happen, it is not that random though, you need a good game. A sci-fy / fantasy on the Gameling usually gives you A LOT of money.
After that you always compete with yourself, your older games. Joystic and everything else (to my knowledge) is just extra stuff that helps your game be better than either your last game or the game you are making a sequel to.
Never release similar genre / topic game after the last one.
Other important stuff
Genre and Topic have to work well together, on the console you are working on, and for the age demographic you are marketing towards.
Each console has certain genre that work well on them and certain age demographics that posses these consoles / platforms.
All of this determines your review score, NOT YOUR ACTUAL SALES.
Sales are modified by sliders, each Genra wants somethings and not others, in more detail on the page I mentioned.
Let's say you want an RPG, now you have to look what consoles work well with RPGs. Everything is good on PC, but the Gameling is VERY good for RPGs.
Now the gameling is strictly for young audience. The PC is very good for mature audience so you could have gone with that, but this example goes with Gmaeling.
Now you need a topic, that works well with RPG, AND is good for a young audience, for example fantasy is all ages.
This will produce a game with good review scores as long as you don't mess anything else up (like your last game being an RPG, or not focusing on the right sliders).
Now sliders (directly impacts sales): RPGs don't want any work on the engine (max 20%), but a lot of focus on gameplay and story (over 40%), a lot on dialog and level (40%) but don't care at all about AI (so any value, but since two need 40% each, max 20%). A lot on world design and graphics (40% each) and anything for sound (20%).
OBSERVATION: you have to fidle a lot with the sliders later on, since medium and up games depend a lot on the distribution of tech and design skills of the people building your game, while in the garage your character is perfectly balanced and small games don't care.
Hope it helps.