I am starting to slowing break into some commercial work. I will be working with a small PR firm who will use me speratically and I wasn't sure how to charge, or what to charge.

Do you charge hourly and if so do you have a minimum amount, say she only needs a headshot or something of someone and it only take 15 mins.

I am very new to this and any advice would be great.

Commercial photographers charge a day rate, which varies based on market - a typical day is 6 to 8 hours, after which you charge hourly overtime. Last month, PDN had an article on "50 photographers in 50 states" that had some detail on what they bill:
http://www.pdnonline.com/pdn/magazine/arti...t_id=1003540788
There are probably tons of other articles on pdnonline that would help you as well.
Also, I would recommend a program such as blinkbid to help with invoicing, so your clients understand there are certain costs associated with what you do. ie) you bill pay for your assistant, rental of gear, usage and licensing/copyright fees, etc.

I don't know what I would charge for one headshot - I would definately make sure it was worth my time, though.

This weekend I took my first set of headshots for some real estate agents, and I didn't know what to charge, so I just asked for $50 a piece. Then after I got done with one of the agents, I was showing her the results in my LCD screen, and she made the comment "Man, you are amazing, I had headshots done before that looked like crap and they charged me $75 for a picture I didn't even like in the end." It was one of those DOH! moments where I knew I should have followed my instinct and charged more. But oh well, we live and learn.

Hope this helps guide you. :-)

But yeah, for other commercial work, like if a magazine or company hired you for a shoot, it's charged by day rates, or half day rates.

I just started doing a gig for my wife's company shooting high end office furniture. But they only use it for a 1 page PDF for the most part, no special lighting or techniques needed. I was doing the printing for them so I knew the quality they expected (original photographer was using a sony prosumer P&S).

I charged them 500 for under 4 hours, or 750 for over, then sold them the shots pretty cheep, $5 for low rez, 20 for full rez, and 250 for exclusive. Considering how little it requires its not a bad deal for either of us. Last job was 1 hour of shooting and maybe 20 minutes editing/sorting/uploading. $500 + images for an off day.