Which speedlight to get?

I currently own a nikon d3000. I do child and family photography (natural light), but have been asked to do a wedding for a friend this summer and will definitely be needing a speedlight for better photos in the church and at night during the reception. I'm learning, but know very little about speed lights at this time and I'm not really sure what to get. Does anyone have a recommendation?

I like the sb800 because it can hold a fifth battery. You would have to get it used.

I suggest you get the SB700 flash and a flash diffusion attachment to soften the light

The SB700 is the lowest cost Nikon dedicated flash and it is designed to work using the Commander Mode on all current Nikon cameras. This allows you to set the flash units output from the camera if you are using more than one flash unit.

Godox tt560 - £25

I'll skip the lecture on not ruining the wedding photos, because I'm sure you already realize that you're in over your head, but you're going to do it anyway…

I'll second Ace's suggestion for the SB700 if you have to watch the cost.

A good and relatively cheap diffuser is Peter Gregg's "Better Bounce Card." It's available direct from Peter or Amazon. I think the "large" size is a good choice. I use that one almost all the time. Here are some samples. Yeah, you can see some shadows when the wall is only a foot behind the subject, but they aren't horrible.

I don't think the D3000 supports off-camera flash, but you can buy a sync cord if you want to hold the flash off camera. The SC-28 costs $50.

You probably will not be allowed to use the flash in the church during the ceremony, so be ready to deal with that.