How do you add an index tag to an existing field of a table?

I can add a field to a table with this;
ALTER TABLE mytable ADD COLUMN newcolumn C(20)

However, either on the same command or on a different command line, I would also like to add
a tag to this field but I can't figure out the syntax.

I don't think I want to use "ADD FOREIGN KEY" because this tries to create a relationship
between this table an another.

Thanks,

Jerryt