AS Christmas approaches, you may be wondering how to decorate your home for the festive season. If so, the YMCA's ''Table Arrangement for Christmas Parties'' course is for you. It is designed for those who want to make their Christmas buffet or long table as tempting as the yuletide fare piled upon it. Techniques for arranging fresh flowers, candles, foliage and Christmas ornaments to best advantage will be highlighted. The course, open to those over 16, will be held every Wednesday between December 8 and 15, from 7 pm to 8.30 pm. The course fee is $120 (member) or $140 (non-member) with another $130 for materials, including containers and other accessories. Participants should bring their own scissors to the lessons. For details, call the YMCA on 369-2211.