First off, good luck in this adventure.

Make sure you don’t have the 18-200 lens lever set to manual focus. Clean the contacts on both the lens and body. If this isn’t the problem send it to Nikon for repair immediately. You have time to get it fixed. How old is this lens? It may still be under warranty. This is the main lens you will use for this shoot...use this lens on your favorite body...

Use your favorite lens on the body.

Have a couple extra batteries for each body.

You can buy a really good Sunday flash from nest buy for 59.00 each. I have 2 of them and they work great. They are powerful and would be good as fill flash. I also have the sb700 flash that I use when I only need one flash. Learn how to use as fill flash. Fill flash really helps in bright sunny situations to eliminate harsh shadows on faces. Make sure you have plenty of extra batteries for these also...you can hope for an overcast day, but I wouldn’t hold your breath.

Enlist someone to be your assistant. Make sure they are familiar with your shot list that you have developed with the bride to be.

One other thing you might want to be sure you do is get a candid or posed photo of everyone attending the wedding.

If your assistant can use a camera set em loose with a camera while you are doing some candid shots as well.

Post some results here. I’m sure others are interested in your results also.

Chuck


You tube and books will help you

I had many broke friends none of whom's parents would pay for their weddings, and they didn't have jobs. Of course, they asked me to shoot their weddings, (and they would even pay for the film!) You have plenty of lenses don't buy more or go to full frame over this! Most of the weddings I shot on a shoestring were with one lens; a 50 f2 H lens on a Nikon FM. That 35 1.8 is the equivalent. If you can't get the 18-200 fixed, leave it at home. Make sure you've got at least a 64gb sd card and a back-up card and battery. Don't mess with a bunch of extraneous lighting junk. Your built-in flash will be fine for fill, just make it minus 1 stop. The 90 might be handy for portraits or from the back of the venue during vows. The 11-16 inside the reception tent.
As an aside, the most valuable part of shooting a wedding I did for my friends, was to go to the rehearsal. That way, I could see what their plan was, and make suggestions in a stress-free venue as to what would be best for the photos, when to shoot what, and where I would be positioning myself for what shot. That way, it was much easier to get the right shots on the actual day.

Nikon users and wedding photographers in general will need to know which cameras you have.

Look here for tutorials: https://www.google.com/search?q=wedding+photography+tutorials+youtube&rlz=1C1CHBF_enUS852US852&oq=wedding+photography+tutori&aqs=chrome.2.0j69i57j0l6.19267j0j7&sourceid=chrome&ie=UTF-8

I have been there a couple of times myself. The last time it was a good friend (my wife and I were married in her beautifully landscaped yard/garden).
Myself and our daughter were asked to do photos, my wife used our videocam to video it and the guests were encouraged to all bring cameras or use their phones. Then everyone put their photos on thumb drives etc and they were all then put on the new husband's computer. He got some editing software and between him and her daughter (school mate of our daughter) and a few of her friends from college here 3 years later they are still finding and doing more photos and videos etc. Between everyone they got 3000+ stills, almost 2000 from myself and our daughter plus about a dozen videos. My daughter did about 1100 of the dinner and dancing that went on for hours and we had a spot we took turns doing posed shots of guests with the bride and groom. And almost all of the nearly 100+ guests wanted to pose with them. The first month they had an album from the shots I and our daughter did. But with so many they just keep finding gems. It has been a bit of an adventure for them.

The wedding(just minutes before sundown with the warm light from the west), meal and dance took place in the same yard my wife and I were married in but it long ago had a pool and concrete deck added. Her home is on a high hill at the south west end with the back yard right at the edge of the very steep south slope so it is lite by the afternoon sun with a mountain range a few miles south across the valley. And when her father died several years ago she found a gardener who maintained the landscaping he had done (gardening was his hobby) way back when she and her first husband bought the house. So trees that had been saplings when my wife and I got married there are now grown but that gardener she found had kept her father's landscaping by replacing the plants when one dies etc. Just that over half of what was grass is now pool and deck. The spot my wife and I were married at is now about the 4-5 foot deep part of the pool.

I'm a hobbyist photographer. My friends ask me from time to time to shoot for pay. I always decline. Instead I offer to do the shoot for free and if they like the photos they can keep them. That's worked out well so far. Somebody wanted photos for their website. Somebody else wanted photos for their horse. All turned out fine. Takes the pressure off me and I stretch myself.

But wait! A young friend recently asked me to photograph her wedding. When my heart rate went back to normal I gave the best response I could. I was honored and would do it if she didn't have alternatives, but this was not a job for a friend. I went through every reason why. Told her about my specific objections and my limitations and that this was not false modesty but real reasons why I wasn't the right choice.
Fast forward. Well it seems that she doesn't have too many options because I just found out her entire wedding budget today and it won't stretch to a pro photographer.

She's getting married on a hilltop with a view for miles at noon in high summer in California. Reception and food outdoors and cake and dancing inside. About 65 people.

I'm going to do my best to accommodate her and I have a couple of months to prepare myself. I have two camera bodies, neither of which are full frame. I have some basic indoor lightboxes and a couple of deflectors. I've never been fond of zoom lenses and prefer prime. I have a Tamron 90 mm, a Tokina 11-16, a Nikkor 18-200 which for some reason is no longer autofocusing (I'll take it in to have it looked at) so I can only use it in manual mode, and a 35 mm 1.8 and 1.4. Looking over my glass inventory I see that I don't have a solid workhorse lens which can be easily adaptable to any situation. I'm willing to buy or rent such a lens. I adore my 1.4 but I don't need the low light feature outdoors. I think my wide angle could be good for a shot of the actual ceremony with about 40 miles of landscape behind it. But really thinking I need to have a goto lens for all purpose shots.
I'd love input and suggestions from people more experienced than myself.
What should I take or buy or use?
I am good with composition and decent in Lightroom. I'm thinking if I shoot in Aperture mode with a good basic lens I can deliver some good shots.
Please don't tell me to just say no. I tried that. I think if I don't step up to the plate we're relying on someone's Iphone. And I'm willing to work to hone my skills.
I have a personal photo blog at ordinarycapture.tumblr.com which should show what I've done in the past. Don't expect to get bowled over.
This person and her fiance are very dear to me and I'd like to step up for them. Thanks to all.