What sort of dog? How old is he/she? What are you having trouble with?
Is this the dog you haven't got yet, or are you planning to get another? If so, I'd wait till your current dog is well-trained before you get another, or you're likely to end up with two problem dogs!
have you enrolled in a training class as this would be the best place to start. there are also many good book in the shops to help but make sure their reward based training as this works really well. you need to decide what you are trying to achieve and stick to the rules you have as like children dogs need to know their place and what you expect from them ie if you don't want your dog on the sofa then never let it on their, any chopping and changing will only confuse the issue. you havn't given enough information for any really exact advice so it's hard to know what help you need,maybe you could explain some more.