It really shouldn't be a barrier. The charity will be looking at a number of things and most smaller charities are flexible and sensible. Generally, as much care is taken with placing foster dogs as with permanently homing them. So, as novice foster carers you are not likely to be landed with a dog with severe medical or behavioural issues. Very often dogs come to rescues because their owners are getting divorced or the number one reason is due to a new baby - the dog is well socialised and has nothing wrong with it. Our own charity that we volunteer for will move heaven and earth to put elderly dogs into foster care rather than kennels especially if they've never experienced them before. In these cases, a loving, sensible home with a warm fireside and regular exercise is the most important thing.
Things that the charity will want to consider and answers you'll need to think about are:- your previous experience with dogs, your house and garden (is it secure?) If you are renting, do you have the consent of the landlord? Are you willing to tolerate possible domestic upsets such as accidents, wees, spilled coffee (knocked over by tails), mud, blood, smells and dog hair? 
You should not have dogs if you have immaculate white carpet and are hysterically houseproud. How many children/other pets? Who is at home at what time? What is the longest the dog will be left? Transport - do you have any? If so, is it suitable for the dog? Do you have any allergies/health concerns?
As a foster carer, you will be looking after the dog prior to its moving on to its forever home, hopefully. You'll need to take the dog as a minimum for basic vet check ups and possibly for other treatment too, such as neutering, chipping etc. So most charities will ask you to report back regularly on things like behaviour, meeting other dogs, meeting children, writing short notes on how they are doing,taking pics or even video footage, possibly taking the dog to meet potential adopters, answering their questions. And of course, when the right person comes along, you'll be letting them go which is always a bit hard, as they usually have made a place in your heart.
Fostering is incredibly worthwhile and I can wholeheartedly recommend it. Good luck and keep us posted.