1. Looking over ur subjects and think of what u really like is a good start. For example, I LOVE maths and I HATE English (primarily the literature aspect)
2. Looking over what u like doing. For example, I LOVE solving stuff - chess, puzzle and I don't really like reading.
3. Picture urself in future....are u working outside or inside? or perhaps are u travelling alot? are u office person? how much income do u like? do u like working alone or with others? do u want to become a business owner or of some sort? For example, I am more of an "office person" but I like interacting with people - communicating my ideas and sharing knowledge and etc.
4. MONEY/job condition...whether you like it or not, u work to earn money. Don't say u don't work for money, EVERYONE DOES. If you are a person who is divided between two jobs - choose the job with better condition and pay (and that's a common sense).
So...for example, considering all that, I want to become an actuary or some finance manager by undertaking actuarial studies. I believe that it will be highly math-orientated which explores great deals of applied maths such as probabiltiy, stats and calculus - which will be FUN compared to other shits like mechanics or projectile motion which i HATE.
Actuary is ideal for me because I believe that u get to interact with people allot esp. explaining complex financial/insurnace models to non-actuaries like stochaistic model. Besides job opportunities are broad and pay is great. For someone like me who wishes to work in investment bank, stock market...it's an IDEAL job. And also, for someone who wishes to pursue fields property investment, commodities, mutual funds, stocks and etc. actuarial studies also works out well.
However, the bottom line is...consider all 4 factors that i put above - there should be WAY MORE. But no one really expects u to choose what u want to become for the rest of ur life. everyone changes their mind but it's always good to choose ur ideal course from the beginning to reduce confusion in later stage of ur life.