There are two fundamental ways of determining your MBTI personality type: The hard way, and the easy way.
The hard way -
You have eight JCF cognitive functions which, when combined in certain ways, somehow create a personality type.
Let's say you determine your primary function readily enough. Now you have to work out your auxiliary.
But wait. Your tertiary can sometimes appear as your auxiliary. How do you know if this is the case for you? You don't, nor can you until you determine which one is your true auxiliary type.
You may say, the difference is that the tertiary of your type has the same attitude (I or E) as the primary. But once you determine that, how do you know if your primary is the correct one? Perhaps you were mistaken from the very beginning. So your tertiary is really your auxiliary, and your primary is some function having the opposing attitude.
Not necessarily, however. What if - just what if - your primary type is undifferentiated due to immaturity or mental illness. In that case, your function order of the moment is determined by your deeper mental/emotional issues which typically come and go.
Now for the easy way -
1. Most of the time you're either introverted or extroverted:
There are different ways of breaking this down; but just for purposes of the MBTI, you're either a type A or type B personality, either generally energized by thoughts, feelings, and ideas, or generally energized by interacting with the external world. Likewise, either too much interaction with the internal world tires you out, or too much interaction with the external world tires you out.
2. You're either the intuitive or sensing type:
No matter what you do with the information you focus on, and whether your preferred focus is internal or external, you prefer to work with and live through either concretes or abstractions. If you prefer to live in a world of concretes, then you will interact with concretes either directly or indirectly. Direct interaction with concretes involves such examples as driving fast cars. Indirect interaction living through the intermediate of externalized symbols or concrete sayings such as "all work and no play..."
If you are the intuitive type, then you prefer to work with and live through a world of abstractions. Compare this to Plato's Ideas: they are in the world, but not OF the world. The intuitive's general goal in life is to interact with abstract notions, and possibly even advance beyond a concern with the material world altogether. As a result, intuitives add novelty to the world. They are different, and they know it.
3. You're either a feeling or thinking type:
You either decide things with your heart or with your head. You either primarily take the feelings of others into account, or you prefer to judge with logic as against feelings. Your first instinct is either to employ justice or mercy.
4. You're either a perceiver or a judger:
Either you prefer spontaneity and excitement; or you prefer control, whether over oneself, others, or both. If you are a judger, then spontaneity doesn't come naturally to you, and when you try to be spontaneous even then it comes off as controlled. If you are a perceiver, then self-control and/or control over others will be difficult for you, and you prefer to live and let live. Moreover, you wish others would see it the same way, and allow you, the perceiver, to live and let live.
Now that you've taken the easy route to determining type, maybe it will help you determine something about your JCF type. However, they aren't they same system, and so they don't always correlate well.