"My I-130 is approved, what is the next step? How will they contact me? I am the beneficiary."
Once your I-130 is approved for a spousal visa, the USCIS is going to send it on to the National Visa Center. Once they receive it, they will send you an email that contains an invoice for the two NVC fees. The first is the affidavit of support fee, then the visa application fee. At the time of this writing, these fees together are $445. You will get a detailed checklist of all the supporting documents you are going to need.
Now you will provide your affidavit of support along with all the documents they tell you to collect. You should not contact the NVC for at least 60 days, as per their instructions. After another waiting period, hopefully near 60 days, you will receive communication and be instructed about what to do for the interview. Don't let it get too far beyond 60 days though. If you haven't heard from them in a reasonable time, contact the NVC to get an update.