The letters read from left to right, and from top to bottom. So in this example:
You can see the first sound (top left) is 'ㅂ', next (right) we have a vowel sound 'ㅏ', and lastly (bottom) there is the final 'ㅇ sound. In this word, the 'ㅇ' has a similar pronunciation to the '-ng' in English.
'ㅇ', however, is only pronounced when it is the LAST sound in a syllable. When you see it first (top/left), it serves another purpose. Here, it is silent, and indicates that the first sound in a syllable is a vowel. For example:
'인', '욮', '을'.
Each syllable may have two, three, or sometimes even four letters. You will start with either a consonant, or the silent 'ㅇ', as explained above. A vowel must then follow. Depending on the vowel letter shape, it will either go to the right of the first letter, or below:
Vowels that are always placed below are: ㅗ, ㅛ, ㅜ, ㅠ, ㅡ
That could be the end of your syllable. Or, if there is a final consonant, it is placed right at the bottom, below the other letters, just as in my first example, '방' (ㅂ + ㅏ + ㅇ).
Other examples: '힘', '촣', 귤', '난'
Occasionally there may be a fourth consonant, and in this case it is placed bottom right. For example, '잛'. In this case you can clearly see that the four characters are pronounced in turn from left to right and from top to bottom: ㅈ + ㅏ + ㄹ + ㅂ = 잛. I won't go into detail about the pronunciation of this kind of syllable in this post, but just remember that you will never see two consonants together at the beginning of a syllable, only at the end.
Don't be confused if you see something like this: '꽁'. That is not two 'ㄱ' letters that you can see, but rather a single 'ㄲ'. The letters ㅃ, ㅉ, ㄸ, ㄲ and ㅆ are sometimes referred to as 'double consonants', but this just describes the way they are written, (similar to how an English 'w' is called a 'double-u'?). They are slightly different sounds to their 'single' counterparts - But again, I'm not going to talk about pronunciation in this post, only writing. Just know that they are different letters.
So the most basic rules to remember are:
1. One 'block' per syllable
2. Read from left to right, top to bottom
3. Two to four letters in each 'block', never only one.
4. Horizontal shaped vowels go below the first letter, and vertical shape vowels go to the right.
5. 'ㅇ' is silent if at the beginning, and pronounced similar to '-ng' if at the end.
6. A vowel must always be preceded by a consonant or a silent 'ㅇ'.