If the tea doesn't have strong enough taste, make sure you are using hot enough water or brewing long enough. In general, darker teas require higher temperature.

If the tea tastes bitter, try to shorten the brewing time or use less leaves. You might have also used too high water temperature.

A good starting point is 4-6 grams of leaves per 100ml water and water temperature from 80 (greens) to 90 (whites and some blacks and oolongs) to 100 (the rest) celsius. Some experimentation is always required. Most greens should handle 85, some even over 90 degrees, while some darker ones won't like over 90 degrees water.

As for the taste, there are many kinds of teas in each category. Two green teas won't taste the same. Chinese greens are usually more toasty, nutty and fruity, while japanese greens are vegetal (grassy) and umami. You'll just have to try different teas to find the ones you like.

When buying tea, buy the smallest amount the seller is selling (usually 10 to 25 grams) so you don't end up wasting money in case you didn't like it. And if available, buy tea samplers which contain several different teas. Samplers are (hopefully) curated by the seller, so those should be good teas in their category. The seller should provide you with information like when the tea was picked (year and season), where it was grown (region and elevation), the variety of the plant, and picking and processing information. If the seller doesn't give any of those information accurately, you should probably give your money to someone else.