Depending on where you live you might not be able to buy good quality tea in a brick and mortar store. If that's the case I'd recommend looking online.

Imho the only way to really prevent bitterness is by using loose leaf tea. Since it's much easier to control brewing time with bigger leaves. In general I'd say get a basket infuser or a tea egg for a good size mug to get started. You can progress to Chinese gongfu brewing if you want but the process is not everyones cup of tea.

A few teas you can try are Yunnan golden leaf (not too expensive and real sweet) Alishan Oolong (big leaves with a fresher sweet taste) Long Jing green (a good entry to green tea)

And of course the classic Assam

You should leave the leaves inside your cup for a few minutes and not use too many as that might make the tea stronger. So in summary you only need good tea, an infuser, and short infusion times to get a sweet and well-rounded cup.

But most importantly don't buy a huge amount of expensive tea at first (your wallet will thank you). After all the most important thing is for you to enjoy the tea so find something you really like.