What background would that hypothetical man need to have, would he have had been a soldier prior to joining, what type of training did he have, and how much training did he receive under the banner of the Templars?
Although I've only really looked at them during their flourishing stages, the Knights Templar were fairly easy to join. The order took certain precautions to ensure they would not become liable for a member's debt, that he was not the member of another order, or that he had used simony to enter the order.
If you're interested in the Knights Templar then a most fantastic resource is available to you in their Rule. This was a (closely guarded) set of rules and ordinances by which the Order was maintained. It was restricted to certain members of the order as once the squires had gotten hold of it and spread it about the town. The best edition is J. Upton-Ward, The French Text of the Rule of the Order of the Knights Templar, (Woodbridge, 1992). The opening of the document is available here which should answer most of your questions about joining (see also the Google Books version for most of the introduction) but the slightly more juicy or esoteric, it really depends on how you feel about being told how many horses and squires each rank of the order could have and how they proceeded in battle, stuff appears far later in the book. Along with many fascinating details their daily lives which have crept into the rules and orders. Well worth investing in (c.£20) alongside one of Malcolm Barber's overview books.
For another view on the Rule as a military manual see M. Bennett's fascinating article here.