Column as an Included column or as a key column

I have a query which has an optional where condition on column classID(not unique). This same classID is being displayed as a select column in various other queries. Currently i have classID as an included column in an index idx,so i think that the idx will be used when classID is in select statement. Do i have to create a a non clustered index for the classID as a key column when the query has where condition for classID.? Confused on this whether to create separate non-clustered indexes for all the where clause columns.?