Your BMR is the amount of calories you need to maintain your current wt. if you are completely sedentary. Obviously, you arent. There are activity multipliers that are used to factor in your daily activities. Once you obtain this number you can add 300-500 calories to start off with to get the amnt of calories you want to ingest per day in order to gain weight.
Of course, this is just a starting point. Read the "Eating for Size" article under the "Articles" tab on this website Everything is there for you to learn, so get to reading! It will tell you all you need to know to get started.
In reference to your T-lvls, the doctor will be able to tell you if your lvls are within normal limits.(If I remember my lab values correctly it should be at least around 250ng/dL) If they are below the low range of normal limits, he or she may prescribe a hormone replacement.