Dont know if this is the right place for this question but here it goes

I have purchased a module for open cart and it tells me to add this

you will need to add a field to the "products" table in the database. Using phpMyAdmin, run the following query against your database (replace DB_PREFIX with your own database prefix):–

But i have no clue where to start.

how did you create your database in the first place?
if you have rights to create tables then you could just as easily do this using code, as you use a dialect of SQL, called DML, to create or modify the schema (the tables)

I'd suggest your first point of call would be to talk to the ISP you use and ask them how to add a new table to your DB. My guess is that they will not allo you to modify the schema using code (too dangerous) and that they will let you use the PHP based tool called PHPMyAdmin

the dbprefix bit is the name of your database on the ISP's server