And these are the laws which you shall set before them (Exodus 21:1)
The phrase "and these" (ve'eileh) implies that they are a continuation of what is written before. This is to teach us that just as the laws written above (the Ten Commandments) are from Sinai, these too are from Sinai.
(Mechilta; Rashi)
When the Holy Temple stood in Jerusalem, each Jew contributed an annual half-shekel to the Temple. The 1st of Adar marked the beginning of the collection of the shekalim. In commemoration, the Torah reading of the Shabbat that falls on or before Adar 1 is supplemented with the verses (Exodus 30:11-16) that relate G-d's commandment to Moses regarding the first giving of the half-shekel.
"Parshat Shekalim" is the first of four special readings added during or immediately before the month of Adar (the other three being "Zachor", "Parah" and "Hachodesh")
Links: The Shekalim Reading with commentary; Partner; Tzedakah: Charity
This Shabbat is Shabbat Mevarchim ("the Shabbat that blesses" the new month): a special prayer is recited blessing the Rosh Chodesh ("Head of the Month") of the upcoming month of Adar, which falls on Tuesday and Wednesday of the following week.
Prior to the blessing, we announce the precise time of the molad, the "birth" of the new moon. See molad times.
It is a Chabad custom to recite the entire book of Psalms before morning prayers, and to conduct farbrengens (chassidic gatherings) in the course of the Shabbat.
Links: Shabbat Mevarchim; Tehillim (the Book of Psalms); The Farbrengen