How to perform full text search

User can enter any number of words as search string.
In shopping cart the following query is used to find products,
eq. if "red cat" is entered:

select * from products
where productname ilike '%'||'red cat'||'%'
or productdescription ilike '%'||'red cat'||'%'
limit 100

This does not find products like "red or black cat".
How to change this query so that it returns 100 best matches for for given search string?
I read documentaton about full text search but havent found step by step solution for this.

Using PostgreSQL 8.2.7 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)

Should I install some contribs to 8.2 or is it better to upgrade server ?

Where to find step by step instructions making this work ?

Andrus.