About toasting in Japan
Usually at formal occassions (a work-related party, wedding, etc.), the head person of the group / company / school (if a wedding, someone close to the bride or groom) will be asked to give a short speech and the toast. When that person calls "Kampai!" and raises their glass, everyone else will do the same; just follow along with everyone else. If you are the person asked to give the toast, you can decide when to call out "Kampai!"