I assume you meant promotion; the term marketing usually includes product (design), price, placement (ie. distribution) and promotion.
Making marketing decisions is basically the same thing as making any decision: You need to determine your goal, identify barriers you need to overcome in order to reach that goal, and then execute.
The difference to technical decision is identifying the barriers. With technical problems, the relevant cause of the barrier, namely the object or software, is under your control and easy to alter. It's also easy to measure the effects of any change. Thus, identifying barriers is rather easy.
With promotional decisions, the cause of the barrier is basically the market, that is: some group of people. They are not under your control, altering their behavior is usually very hard, and measurement is an art form since you often don't know who they are. Therefore, identifying the relevant barriers is often a mix of experience, intuition, and guesswork.
Of course, there are lists of common barriers (ie. causes why people don't buy or buy elsewhere) and for each of them, there are common methods to overcome (or answer) these.
For example, a game is a typical experience good: you can only judge the value after having played (and payed for) the game. Thus, mistrust is a likely barrier to overcome. Typical answers are offering a trial, a demo or a guarantee, setting the price to zero, etc. -- the "low commitment" route. A different route could be using "high-reward" signals: promoting the (high) production costs, setting a rather high price, producing high-quality in-game videos, buying TV time for ads, offering trials to only selected journalists, etc.
As you can see from the example, money may be a necessary ingredient, sometimes lots of it. Thus, promotion is also a function of the price (or rather, the revenue model).
However, since you seem to go the "low commitment" route, already, your central problem will be "selection": Why play your game instead of one of the million others?
A common answer is "relating" or "bounding" -- appeal to something a target audience wants to be (or thinks it is). For example, Listerine's 'Always a bridesmaid, never a bride' appealed to what the audience wanted to be (namely someone's wife). Marlboro's 'Come to Marlboro country' appealed to freedom and independence. So, another ingredient is creativity, so to say.
If you have that, your promotion only needs to communicate this particular trait. The rest is finding out what people do, view or read who should be interested in this particular trait and go for it.
Then, you really just need time and effort to promote your product successfully.
Hope this helps.