Mind, please, PLEASE don't give up. Fostering is one of the most important elements of rescue work. 
By fostering you will be giving a loving home to a dog that might not be able to cope with kennels (many dogs find them immensely stressful) and they are not ideal for older or poorly dogs. Also a domestic environment is great because unlike kennels, the foster home can provide a really good assessment of how the dog behaves in circumstances that will be of interest to the adopter e.g. is he housetrained, how is he on walks, does he like playing with toys, etc etc. Because you can provide this info, the rescue can then give a much more detailed assessment to adopters, and dogs who are in foster care ALWAYS get adopted faster and easier than ones in kennels
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Most rescues are desperate for foster homes - usually a reputable one will start by getting you in touch with a foster co-ordinator who will discuss with you your experience, expectations, domestic arrangements etc. As Dooin says, they will want to match you with a nice easy dog on your first foster, to see how things go. As your skill develops, you MAY want to (but are under no obligation) to take on slightly more complex dogs - perhaps ones that need nursing after an injury, for instance, or have special needs. But please don't think you have to - most foster dogs are just unlucky enough to have been surrendered by their previous owners, mostly for reasons nothing to do with them e.g. arrival of second baby, marriage breakdown, redundancy, etc. You will see plenty of threads here on the Doghouse, where people will be wanting to give up dogs for various reasons. It's amazing how often allergies develop when a second baby comes along 
Our local (S Wales) all breed rescue is Hope Rescue who are excellent and have foster homes all over the country. Click here to visit their website/forum which gives loads of info and has a downloadable foster application form. Four Paws Rescue is also very good.
Hope it all goes well for you. Fostering is so rewarding and fun, and it is the most wonderful feeling in the world when you see a dog that you first saw as a wreck later on when they are happily rehomed, well and flourishing. Cue huge lump in throat. It's also about helping people - for instance two of our current foster dogs are here because their owner became very unwell and has been in hospital. Knowing her beloved dogs were being well cared for, together in a greyhound knowledgeable home gave her a huge feeling of reassurance and she could concentrate on getting better. We've even been able to take the dogs to visit her in hospital which was lovely.
Good luck! 