If you have a primary key set on that table then a clustered index will be created .The other columns which are widely used in the queries and which are used in the 'where' clause you can create non-clustered indexes on them.But the creation of indexes should be balanced and you need to optimise it by testing .More number of indexes results in slow inserts and also increases the maintenance costs.
You need info about different things before deciding on the indexes. Some of them are
Volume of data in the table (IMP),
Type of queries that are going to hit the table (IMP)
Purpose of the data , Connectivity , Concurrent connections, Server Hardware.....
There are other considerations also but offhand i can think of these.Of these the first two are most important and index design is influenced the most by them.
Clustered index keep the actual data in the leaf pages of the index in a sorted order. In many of the cases clustered index also functions as the Primary key/unique index and so you would want your most "selective" columns to be part of the clust index.
best practices are
* frequently used tables must have a clustered index
* no need create index on tables having little records or columns having duplicates (like country, gender etc) with exception if the columns are filtered on often it may be very beneficial to have an index on the column even if there are lots of duplicates. The index may need to be covering for SQL to use it.
* avoid duplicate index sets like (col1,col2, col3) and (col1, col3,col2)
In above situation, they must have the same columns in the same order. There are queries that could effectivly used the first of those indexes, but not the second and vis versa.
* create indexes on columns which are frequently used in WHERE and JOINS.guidelines for selecting clustered indexe key
* columns that contain large number of unique values
* queries that returns range of values.
* columns that are accessed sequentially
* queries that returns large resultset
* columns which are not updated frequently.guidelines for selecting nonclustered indexe key
* queries dont return large resultset
* columns frequently involved in WHERE having exact match