I'm afraid that I've never read into the practical workings of the Knights Templar, so I can't say that I know of any sort of Templar "recruiters." The Knights Templar grew very quickly, and at its height had bases across Europe where people could sign up at.

Your father wouldn't be able to sign you up, though, if you were a child. The "Primitive Rules of the Templars," that is to say, the rules that the organization followed, forbade that:

14 - Although the rule of the holy fathers allows the receiving of children into a religious life, we do not advise you to do this. For he who wishes to give his child eternally to the order of knighthood should bring him up until such time as he is able to bear arms with vigour, and rid the land of the enemies of Jesus Christ. Then let the mother and father lead him to the house and make his request known to the brothers; and it is much better if he does not take the vow when he is a child, but when he is older, and it is better if he does not regret it than if he regrets it. And henceforth let him be put to the test according to the wisdom of the Master and brothers and according to the honesty of the life of the one who asks to be admitted to the brotherhood.

It's a rather sensible rule, given the sort of organization the Templars were. The Order of the Temple was a religious order, but seeing that it was a religious order that required its men to be capable of fighting in battle, it is understandable that they wouldn't want to be stuck with raising children.

As for how adults could join, this bit covers that:

11 - If any secular knight, or any other man, wishes to leave the mass of perdition and abandon that secular life and choose your communal life, do not consent to receive him immediately, for thus said my lord St Paul: Probate spiritus si ex Deo sunt. That is to say: 'Test the soul to see if it comes from God.' Rather, if the company of the brothers is to be granted to him, let the Rule be read to him, and if he wishes to studiously obey the commandments of the Rule, and if it pleases the Master and the brothers to receive him, let him reveal his wish and desire before all the brothers assembled in chapter and let him make his request with a pure heart.

Seeing that in this hypothetical scenario you are the second son of a noble, if you were of age to join the Templars, you would probably be admitted unless you had some sort of disability that would prevent you from being an effective knight. "If it pleases the Master and the brothers" is a vague rule, which allows for Templars to make case-by-case judgements on whether they want to accept a volunteer or not, but the son of a lord, who would have likely had access to military training from an early age, could probably get in relatively easily. Actually serving as a Templar is another matter, of course - the Rules are pretty strict, reflective of the organization's nature as a religious order.

Even excommunicated knights could join the Templars, provided that they got the approval of a bishop.

Here are the full Rules of the Templars:

http://www.the-orb.net/encyclop/religion/monastic/t_rule.html