Typically this will consist of a phone number, an e-mail address, a website, and maybe the address of your office. Here your number, email and website are most likely to be the most important details as they give the user everything they need to obtain more information regarding your service. Decide whether you want this to go around the logo/company name, next to it, under it or on the reverse of the card. Of course you can get a more eye catching look if you opt for the back of the card, but you will need to ensure that your business cards printing service prints on both sides which will cost slightly more, and will at the same time risk the holder of the card not realising they need to turn over.
Another thing you may wish to include is a tiny bit of blurb about your company, just in case your recipient forgets the reason they have the card (or in case it's been passed on further or found by someone else).