My hair is about 7 to 8 inches long, and it can kinda puffy from the back.

It's not really that much of a problem, but if there is something out there, I would buy it. So I was wondering if there is a product like a conditioner that can make it more straight and with less volume(if it's possible I guess). I don't know much about conditioners either since I don't use them.

Use a conditioner. (especially if you are looking to highlight your hair. Dye will FRY hair that isn't properly conditioned) It will definitely make your hair less "puffy". That's one of the biggest reasons people use conditioner. Suave BioBasics has worked okay for me, and some good reviews have also been posted for Biolage Conditioning Balm.  Hope this helps.

Anais

You could also get a haircut that reduces volume.  It may be hard to find a stylist who can do this, try looking for someone with naturally curly hair themselves.  Also, AnaisSatin was right on about conditioner.  A good heavy conditioner will weigh your hair down more.  Try using a gel through your hair also will help with the puffiness.

Some good leave-in conditioners are L'Oreal Natures Therapy Heat Control, Neutrogena Triple Silk, Pantene Repair and Restructure.

I use Ouidad Climate Control heat& Humidity Gel.  L'Oreal gels and Suave gels are good too.