Immune System Question

While reading my immunology textbook there is a lot of talk about how all T/B-cells and antibodies are made specifically for one antigen. It however does not describe how the original antibody/cell produced or got that specificity. The book said there are random mutations when they are produced in bone marrow, but does that mean that all your antibodies are made randomly and are only mass produced when something matches up to them?

So to cut to the chase how does your body make new t/b-cells and antibodies for a previously never before encountered antigen?

Thanks in advance!

edit: Also if that is true about the mutations does that mean that all the antibody variations you will ever have are just sitting in your blood right now?