Theres tons of questions to ask but most of them you will need to ask yourself first: what size dog do i want?
how big is my house?
how long each day will my dog be left alone?
will i be able to offer enough walks and play times?
can i afford a ton of vets bills and insurance?
will i go to training/puppy classes
do i want a puppy or an older dog?
does it have to be pedigree or will a mix suit me?
the list goes on and on.
once you have an idea of what you are able to offer your dog then you need to think about what the needs for each breed are and if they fit your needs. it's no good falling in love with a great dane if you live in a one room flat in the middle of a city and work eight hours (bit over the top but do you see what i mean?) where as if you live in the country and have a massive garden and are at work for just part of the day things differ and you have a different choice of dogs. so much to think about
good luck and don't get brain strain thinking about it just enjoy the fact you may soon have a four legged friend