A reputable rescue will advise you and do a great deal to assess you/your family/your home/existing dog(s) in the same way they would for an adoption.
A foster carer will give a detailed assessment of the dog's character and personality thus enabling the rescue to place the dog with the right forever home. Sometimes a foster carer is responsible for nursing a dog back to health before it is well enough to be adopted though it is unlikely when it is your first and I would be very 
if they presented you with a dog with complex health needs. If the dog has not previously been spayed or neutered, this is often done while in the care of the foster home, as especially for bitches it's important to have TLC when recovering from invasive surgery. A foster carer may be the first time a dog has lived in a domestic home, so you may be introducing them to things like the hoover, glass doors, stairs, etc and you may be responsible for toilet training, and beginning other training.
However, there are no hard and fast rules as so much depends on the dog and the rescue you are doing it for. What you should have made clear to you, is the system for reimbursing you for feed and any other expenses such as travelling to vets, how vets treatment is authorised (and where - if they have a specific practice), who you can contact for support and what assessment has been done prior to the dog arriving with you. If the placement does not work out (which they sometimes don't) they should advise you on what should be done. For the dog's safety they should give you very clear guidance on things like not letting the dog off the lead (at least for some time) or unless you are absolutely safe (e.g. your own fenced paddock), checking the security of your garden, and making sure that all family members are briefed on door safety i.e. not leaving any open that could allow an escape. Foster dog should be chipped and wormed asap and you should make sure dog always wears ID. Dog should have an appropriate well fitting collar and lead (NOT an extending one - the devil's work and lethal if you have a sighthound). Check also about whether dog needs any coats, blankets, beds, dietary requirements etc.
Usually, unless it's already been done, you will want to take dog to vet for an assessment and this is often a good time to get chipping, worming, flea treatment under way. The assessment is also a good opportunity to get a handle on what if any vet treatment will be needed, apart from spay/neutering - dental for instance.
Later on, you may be responsible for transporting dog to meetings with potential new owners. You will provide things like pictures and videos of the dog for the rescue's website, and provide text to describe the dog to a new home. Sometimes (not always) you may be invited to give a view on the suitability of a potential new home.
Once they are adopted, it is very difficult emotionally to let them go and most fosterers (including us!) fail at least once and end up keeping a foster dog. If you have DC you should be prepared for this which is emotionally very difficult when a bond has formed. However, it is the most extraordinary feeling when you know a dog has gone to a loving new home - very satisfying and exciting. You will feel very proud. If you are very lucky you may keep in touch with the new owners (especially if they are local) and that is a wonderful thing too. To see a dog that you first saw as a pathetic bundle of bones, now as a happy, healthy, loved family member is one of the loveliest things in the world. It is a great privilege. Good luck!