You need to think about what you can offer in terms of space, garden, exercise, time alone, training, lifespan. This will help you narrow down breeds.
Do you have the time/patience for a puppy, or is an older dog better?
Then think about what you are going to do with the dog whilst you are at work, going out in the evening, weekends away, holidays etc.
Look at insurance costs, vet practices near you.
Then look at some rescue centres near you, explain what size and excise needs you are looking for.
Don’t just jump in for the first dog suggested and the one hat looks he saddest. You need to find the one that will fit in with your life the best. Don’t be afraid to say no, don’t be afraid to ask to go away and think about it, come back for multiple visits etc.