Well my petition took 165 days to be approved. I got an RFE and did it all myself. You can do it yourself too. First when you receive the letter read it and see what they are looking for and if there is something you don't really understand post it here. I'm sure anybody can help you. It's just a way to save money, because many people do it without the help of lawyers. I'm not telling you not do it, but i'm saying that doing it yourself is also a good option. This site is sooo helpful that simple cases should not required the help of lawyers or the money that you will later need for other expenses with your loved one!
Good luck and keep us updated!
Your US citizen spouse files the I-130. You file the I-485. It's best if you file them both at the same time, and in the same package. Harpa Timsah gave you a link to the step-by-step procedure you should follow. Read the information there and follow the instructions.
There are a number of other forms you must also complete and submit. Some of these forms require additional documents and evidence. Make sure you read the instructions for each and every form, and include everything that's required in your package to USCIS. Filing the petition has the effect of calling the attention of USCIS to yourself. If you are denied because you didn't prepare your forms, documents, and evidence correctly then you could find yourself appealing the decision in front of an immigration judge while trying to stop USCIS from deporting you. This would suck big time. Take your time, read everything, and get it right the first time.