What exactly is negative and positive selection?
Negative selection, also known (in one form) as clonal deletion (VCE level), involves the elimination (via apoptosis) of self-reactive B cells and T cells of the immune system. This occurs within the Thymus for T cells and the Bone marrow for B cells. Ultimately, what occurs is that self-antigens are presented on the MHC of the cells in the bone marrow or thymus and if the respective B and T cells are reactive toward such (generate an immune response), they are eliminated (so as to not cause an autoimmune disease in the host).
Now, positive selection involves the recognition of non-self antigens presented and generation of an immune response. If an immune response is generated by the immune cell, it will not be eliminated, if not, the contrary will occur.
Ultimately, both B cells and T cells must undergo these tests in the primary lymphoid organs and "pass" (mostly*) to be considered naive and self-tolerant.
*cells that demonstrate a moderate affinity toward self-antigens still pass the negative selection process.