I was a noob the first time I did it. The most important thing is to trust your intuition. If it feels cleared then it's cleared. If it still feels dank, dingy, thick, heavy, or smells bleah, then it's not cleared and you need to keep working on it. You will be surprised how much better the place is already after you take possession of it and give it a good clean through. I imagine it would be an expensive process to get a 'professional' through - it might be worth giving it a go yourself and then if you're still not happy with the results, then think about getting someone in, kwim?

I will PM you some links.