First make sure the column(s) are of type varchar or text. If not then you need to alter the table and change them to one of those data types (eg. ALTER TABLE table_name MODIFY column_name varchar NOT NULL). Then type this query...
ALTER TABLE table_name ADD FULLTEXT (column_name)
... Of coarse you should replace table_name with the name of your table and column_name with the name of the column that you want to have indexed.