Diction is just a fancy way of saying 'word choice.' That is, when someone tells you that you have 'good diction,' they're saying that you have a good vocabulary and you use it well. Having good diction is an essential part of writing well no matter what kind of writing you're doing. Diction is the key to developing your style, tone, and point of view.
Don't Use Uninteresting Words.
Whenever you can, try to replace boring words with more interesting (and descriptive) ones. Take this sentence:.
'James Blonde's good plan helped him win against the bad guy.'.
That gets the information across, but it doesn't pop. Let's target these words: 'good,' 'plan,' 'win,' 'bad,' and 'guy.' Instead of the adjective 'good' we can substitute in 'cunning' (which means clever), instead of the noun 'plan' we can say 'strategy' (which has a similar meaning but implies more action), instead of the verb 'win' we can say 'defeat' (which is more dramatic), and instead of 'bad guy' we can say 'villain' (which sounds more threatening and thus makes James Blonde's victory more epic).
So what are we left with? 'James Blonde's cunning strategy helped him defeat the villain.'.
Make just a few small changes in word choice and the sentence now has much more zip. However, that doesn't mean you should replace 10-cent words with 10-dollar ones just because you can. That's to say, just because a word sounds fancier (e.g. it 'costs more') doesn't actually make it better. For instance, 'James Blonde's dexterous machinations abetted his vanquishment of his disputant' is full of big, heady words and is also confusing and totally awful. Good diction requires you to strike a balance between the dull and wordy while still sounding natural.
Specific Details Are Better.
Replacing boring words with more interesting ones is good; replacing vague words (which are usually boring, by the way) with more specific ones is even better.
'It was a nice day.
