Non-diabetic usually have a fasting BG of less than 5.5. Anything in the 5.5-7 range is generally considered to be pre-diabetic and 7+ is usually indicative of full-blown diabetes. Most doctors like to confirm this with a Glucose Tolerance Test, although but two readings of over 7 on separate days is fairly conclusive.
There is no way of telling how long you have had diabetes, although if you have no symptoms then it is probably either fairly mild at the moment, or else you have not had it very long. Although your figures are in the diabetic range, they are quite low by the standards of uncontrolled diabetics. Because mild diabetes doesn't tend to cause symptoms, it sometimes takes many years before it is diagnosed. However, the important thing is that if it hasn't caused you any problems yet then you can control it and should be able to prevent such problems from occurring.
What to do now. First of all, don't panic! Diabetes is a nuisance, but it isn't the end of the world - it is quite possible to control it and still lead a normal life. If it is Type 2 diabetes there is every chance that you will be able to control it with diet and exercise, but if that doesn't work then there are a number of very effective drugs. Secondly, the most important thing that you need to do is to start learning about the condition. This web site is a great place to start - read through past discussions and remember that the only stupid question is the one that isn't asked! Talk to your GP and your diabetes nurse - they will, no doubt, give you plenty of advice. You might like to get some books on diabetes - see discussions on here for some ideas as to which ones are worthwhile or post questions.