Is it good to get married in the month of Nisan, before Pesach? What about Rosh Chodesh Nisan, would one have to fast?

There is no problem to get married from Rosh Chodesh Nisan until Pesach. Regarding a wedding on Rosh Chodesh night itself, some places have a Minhag not to have a wedding before the new moon is born, but places that don't keep this Minhag don't need to follow it. Regarding fasting - Sepharadim do not have a custom for the bride and groom to fast. Ashkenazim fast on these days, as is the Rma's opinion, except for Rosh Chodesh itself, when they don't, unless they follow the Minhag to every Rosh Chodesh Nisan, because of the death of Aharon's sons. Rabbi Ro'i Margalit