The Selichot that we will begin reciting this Motzaei Shabbat. When must we stand? When must we sit? When it is optional?

In general one should stand when all are standing and sit when all are seated. It is important to be an integral part of the community even in these unimportant practices. The customs vary from Shule to Shule and community to community, some have the custom to stand all way through, others will only stand during the 13 attributes of mercy [13 Midot] and when the Arc in opened, any other custom in between these can also be found. In a case a man can't stand for medical reasons or he is too old to be on his tired feet, he can spend all of Slichot sitting on his chair.