Why loose leaf tastes different
Most tea bags contain cut or CTC leaf, which is small, uniform and built to extract fast and completely in two or three minutes. Whole leaf extracts slowly and incompletely, which is what allows a garden, a season and a cultivar to be tasted at all. The trade is real in both directions: bags are reliable and fast, loose leaf is more variable and has more to give. Neither is a compromised version of the other.
How much to use
A standard tea bag holds about 2 g. Loose leaf at 2 to 3 g per 200 ml sounds equivalent, and by weight it is — but it looks like far more, because whole leaf is much less dense. A teaspoon of tightly rolled gunpowder and a teaspoon of fluffy white tea differ several times over in weight. Weigh it once, look carefully at what that quantity fills, and then use your eye. Almost everyone starting out under-leafs, then concludes loose tea is weak.
Give it room, or skip the basket entirely
The basket that came with your mug is very likely too small. If the leaf cannot open, most of it never contributes. A large basket, a pot with an integral filter, or a strainer held over the cup while pouring all solve it. So does grandpa style: put a small amount of leaf straight into a tall glass, top up with hot water, and drink around it. That approach needs no equipment at all and is how a great deal of tea is drunk in China.
Take the leaf out — and then use it again
A bag gets thrown away. Loose leaf usually should not be, at least not immediately. Most whole-leaf teas give a good second infusion and many give a third, and the second is frequently the best. Take the leaf out when the steep is up, put it somewhere it can drain, and come back to it within a few hours. Leaving it sitting in water is what makes the last cup bitter.
What to start with
Something forgiving and cheap enough to experiment with. A robust black tea, a Chinese green such as Longjing, or a rolled oolong like Tie Guan Yin will all tolerate a rough first attempt and reward a better second one. Buy small quantities of several teas rather than a large quantity of one — tea does not improve in an opened packet, and the point at this stage is finding out what you like.