I've noticed that we have a non-immigrant visa thread here (started by Lady G) but none specific for Immigrant visa questions (apart from the one by the VO)
Looking through, I've seen threads for DV Visa but they are specific to the visa lottery, which Nigeria is no longer eligible for.
There are some questions that are better answered by experience so I think this is necessary. If a moderator thinks differently, no wahala.
In the meantime, I'll pull up a couple of questions and try to answer them as best as I can

YerYer:
Hi my fiancee who's in Nigeria has been asked to provide a police certificate which should be brought in on the day of the interview. Please what are the steps involved in getting a police certificate in Lagos and how long does it take? Also, she's supposed to have a medical exam done at one of the clinics recommended by the U.S embassy; I called the listed phone numbers to the clinic and no one picked, I just want to know how easy it is to get an appointment or do you just show up to the clinic and have the medical examination done there and then? Can you also, shed some light on some of the common questions asked during the interview for K1 visa. Thanks

1. You can obtain Police Certificate at Alagbon Close in Ikoyi. Once you get there, just ask any of the officers, they'll direct you to the proper officer. It takes a few hours.
2. You're probably calling the hospitals at the wrong times or when the lines are busy. If you can't get them on the phone, tell the person to just go there. They accept walk-ins too. Budget like half a day. Pick up of final results takes about a week

Following.

imnuflawal:
Hello CO, pls I want to confirm this sir, i've been issued my visa but I just wanna ask if it's compulsory I pay for my green card before leaving for the US or I can pay it on my arrival to the State, tanks for ur concise and most accurate response...

Better to pay well before you travel. The physical green card takes long enough to get here. The earlier you pay the quicker your card can be processed.
Nothing will start at USCIS until you pay the fee. And arrive in the US. So don't wait