Samithadhanam is a daily ritual meant to be performed by Brahmacharis who have had the sacred thread, poonal, done for them.
A relatively simple, short ritual, invokes Lord Agni to bestow good things and protect. The central part of samithadhanam is offering samith to Lord Agni in homa gundam along with mantras.
I have personally immensely benefited by it in my younger days. You can refer to the section samithandhanam in the sandhyavandanam book by 'Anna' of Ramakrishna Mutt publication. That takes you through the entire procedure with meanings. Alternatively, any pamphlet distributed during Avani Avittam describing the rituals will contain this. I would recommend the book by R K Mutt.
It would be advisable to find a knowledgeable vadhyar to guide for a few days and then one can do on one's own.
