What can I do to lower blood sugar immediately? And why do people feel weak and tired when theyre sugar goes high?

Talk to your doctor. Get a plan for correcting high sugars. One easy thing to do is drinking water. Not soda, juice, or anything else - just water. Anytime your sugar is over 200, you become dehydrated, and just drinking water can lower your blood sugar quite a bit. Light exercise such as walking at an easy pace may help the muscles pull more sugar out of the blood. Avoid eating carbs until your sugar comes down.
Insulin? Rapid acting Insulin is the most effective way to immediately lower sugar, but is not necessary or appropriate for everybody. People feel weak and tired when their blood sugars are high partly due to a certain degree of dehydration but also because the glucose is not getting in the cells where it can be used as energy.