B -Cell activation by helper T cell
Naive T cells hang out in the lymph node waiting for antigen presenting cells (APCs) like DCs to present processed antigens on their MHC molecules. Naive B cells similarly hang out in the B cell zone and are capable of interacting with other T cells and follicular dendritic cells (FDCs). Antigen that ends up in the lymph ends up traveling to the lymph node, where B cells internalize, process and display them on MHC molecules. This is because B cells are also APCs. 

Activated (and differentiated) T cells that were previously activated by DCs can identify some of these B cells clones through the TCR-MHC interaction. The B cells process the antigens like DCs, so the resulting heterogeneity of the processed peptides means a fair chance a T cell will recognize it. The result is that B cells receive an activating signal through the BCR and costimulatory receptors. 

Differentiated T cells also secrete different profiles of cytokines. For example, Th1 cells secrete IL-12 and IFN-y whereas Th2 cells secrete IL-4, IL-5 and IL-13. Not exclusively, just predominantly or in greater amounts. 

The receptor signaling in conjunction with certain cytokines can induce class switching to the resulting antibody production. This swaps the constant or Fc region of the antibody to a different isoform. So Naive B cells are IgM/IgD, and if the cells is receiving Th2 cytokine stimulation, class swtiching results in IgE antibodies, for example.

For elementary immunology I like to reference Cellular and Molecular Immunology, 8th ED. by Abbas et al., but 9th ED is out so keep that in mind. The field somewhat rapidly evolves.