Why does the Torah add the prefix 'Mem' in the word "me'Reishis"?
Rashi: To teach us that one only separates a small Chalah from the first dough to give to the Kohen, and not the entire dough. 1
See Torah Temimah, citing Yerushalmi, Chalah, 1:7.
And what are thee ramifications of the word "Titmu la'Hashem"?
Rashi: It implies that, even though Chalah has no Shi'ur, one should give the Kohen dough that is sufficient to be considered a 'Nesinah'.