Well first I would like to say welcome to TC TEACHAT your sure place for tea info around the clock
Anyways as far as your problem it could be a numerous amount of things. Most likely I would say you are doing one of three things wrong.
1. How hot is the water you are using. Typically if you want your oolong to be really robust you should use hotter water. Maybe 180's or 190's but not quite boiling.
2. Length of steeping time...Usually the more robust, the longer the time possiably 3 or 4 minutes at max but if it starts tasting the slightest bit bitter you know you have gone tooooo far.
3. Amount of tea....again more tea means more robust of a flavor but if it starts turning bitter adjust accordingly.
And most people on here would kill me for suggesting this butttttt sometimes adding sugar will help add body to the brew. Shhhhhh don't tell everyone else I said that
As far as your equipment. If you have a kettle use that to heat up your water, otherwise you can use a microwave if you want. And if you have a strainer just put tea in that, put the strainer full of tea into the mug of hot water, then lift out the strainer when the tea is done brewing and WALAAAAAA a nice cup of tea.
Hope this helped a little