Most of us find that blood sugar levels are directly related and attributable to what we eat. Generally speaking your BG should be lower before you eat a meal, then rise gradually as you digest that meal. Depending on what you ate, your peak highest level is going to be something like 1-2 hours after eating. Then it should start to drop again heading towards your next meal.
Ideally, under really good control, you'd hope that these numbers would only fluctuate over 24 hours by around 3-4mmol/l between lowest and highest readings. Most of us probably find the range a bit wider than that, but significantly wider would need something to be changed.
For example, I've gone on insulin recently as my numbers were both very high and fluctuating wildly. I've now found, along with reducing the amount of carbs I eat, that my range is typically around 4 at most - largely because on the days I exercise well I can drop one of the readings significantly, widening the range. On days where I'm not as active, the range is only 2-3 - although I'm still a bit high overall. But last night, I had a dinner with too much potato (where I've eaten very little recently) and it shot up by around 5 for an hour - no one to blame but myself for that one.
So even if you're on medication to control your diabetes, you still need to be mindful of what you eat - so the first thought is that you're simply eating too many carbohydrates that can raise blood sugar - lowering the carbs eaten can bring it down significantly - the medication alone won't control it, you need to make some effort yourself too.
What typically do you eat in a day - if you give more details, we can be more specific in our advice.