Before Apply Indexes on any Table's Column go through some points

1.) Table Type: If your table is very highly transactional than try to keep less indexes on that table
                 that doesn't mean that don't go for indexing. But for selected columns only because
                 indexes provides fast searching data but slow down the insert and update operation.

2.) Table Data: If Table having lots of rows or Data than indexing is good for searching but if your
                 Data is less than it goes bad.

3.) Indexed Column Data Type: Try to Keep Column ;in which you are going to apply index; Data Type should be
                               small in terms of size

4. Choosing Column For Indexes: Try to apply indexes on those column whose will use in joining filtering or
                                 searching records.In other word you can say if you are apply FK on any column
                                 and it will frequently be use in joining than you can use indexes on it.

For more details plz follow this link: http://shivendrayadav.blogspot.com/2011/11/indexes-on-tables.html