My advise would be to test at least 4 times a day to start with, as you get used to testing you might find you can test less. The fasting test first thing, before your main meal and then 2 ours after, and before bed. You might want to test before and 2 hours after each meal to start with just to find out how things effect you.
My fasting level has become pretty stable so I just test that every so often, about 3 consecutive mornings every 2 - 3 weeks to give me a basic idea of how its doing. Testing when I get in from work acts to make sure I don't snack too much at work (you wouldn't believe how much food gets offered around the office) and works as the pre meal reading as I tend to start cooking as soon as I get in. 2 hours after eating, and before bed just make sure I don't go overboard on portion size or on my evening snacks.
You know your daily routine best so you can best decide when to test or not it should just act as a guide to how food effects you.
If you have an unusually high or low reading, first of all don't panic, wash your hands well again and re-test. it is possible to get a rogue reading. If the reading is still the same then I would guess it partly depends what medication you are on, I'm only on Metformin, so can only say what I would do, if you are on other meds it might be different. For a low reading I would either eat a small amount of fairly quick release carbs, the ubiquitous jelly baby or similar, or if I wasn't feeling bad try to wait it out, my body can normally cope with low levels by releasing glucose. For high levels, drinking plenty and exercising can help bring it down, testing again after another hour should hopefully see it start to drop, but if you have had a meal with a high level of slow release carbs it might take longer. Try and avoid eating any more carbs until it comes down to a reasonable level. ( I was out the other day, and ate out, I knew the meal would send me high the moment I tasted the sauce even with all the walking I had done. Two hours after eating 10.2, at 3 hours 10.6, at 5 hours, just before my next meal 8.4, I had a virtually carb free meal, and two hours later down to 6.5, not levels I would want to go to often, but occasionally I don't think will hurt too much, the main thing was to give it time to recover afterwards.)
If it stays high for a long time, or just keeps rising, you might want to call NHS direct or your doctor for further advise.
Learn to be guided by your meter, but don't let it rule your life.