Add a few extra fields to write post page?

Hi, i want to use wordpress to advertise events in relation to my organisation. To do this i’ll need a few extra fields to the post form; venue, and time of the gig. I am reasonably comfortable editing php- but i don’t want to go around “breaking” the script without some guidance.

Which files will i need to edit (i’m not using a template to display the data, i’m collecting the data directly from the MySQL database)