Hi! You need to collect whole blood in heparinized vacutainer tubes. If you perform a density gradient centrifugation with Ficoll 1079 (e.g. Sigma Histopaque, Hypaque or similar at 1.079 density), you greatly enrich for lymphocytes (T and B cells). To further purify, one of the easiest ways is to use Miltenyi's MACS human T cell negative selection systems, or other column based systems like those from R&D. You can also FAC sort your cells. A combination of density gradient +MACS usually gives >95% pure T cells, which can be used for a downstream assay.
Detailed techniques for mouse and human T cells are present on this site (use the Solutions Search tab above) - the main differences between mouse and human are in the collection of cells, density used in the gradient, and Abs/kits used for final purif.
All the best
-sam