A canon rebel t3i, 50 1.8 and the kit lens is going to make taking on weddings really hard. Typically they don’t allow for flash photography, it’s best to plan on NOT being able to use a speedlight and being surprised if the Officiate allows it.

Is this wedding a church wedding, if so I’m going to go right out on the limb and say it, you do NOT have the equipment to even start thinking about taking on weddings, even for free, pass on the event and go as a guest if invited.

A starter kit for a wedding photographer looks something like this:
2 camera bodies
17-55 cropped body, the 24-70 for FF
70-200
A couple of strobes, umbrellas and light stands.
extra batteries and memory cards
tripod
triggers to fire lights

not needed but nice to have would be a macro lens for detail and ring shots.

You might be amazed - if you contact a well established overloaded studio - they might be willing to pay you a flat fee per wedding to assist and learn what they are doing. Many Pro Studios are tight others wanting to ad some part timers - that get good enough to fill in for them when on vaction or out of town assignments. The way I got my foot in the door was I was in a Spring Art Show with entries in Oil Painting, Water Color and Photography - my three favorite 2D worlds. The photographer covering the event for the news paper had a very successful studio. I took 2nd place in photography, and he invited me to come work with him that Summer - his entire family was involved in the Studio Business - his wife taught me retouching, he let me lose in the dark room, as well copy work for this lawyers office using the 4x5 Deardorff photographing big fat checks for record on cut film. By the next Summer he let me shot side by side in a wedding moching every move he made. When the prints came back from the lab - he pulled nearly 50% of my prints to put in the clients album - the he told me that he and his wife had not had a Summer Vacation in years - so he told me to do the next two weddings on my own and help Grandpa and the kids run the studio for the next two weeks.
My stomach began to turn - I asked do you really think I'm that good - his reply - I'm sure you are going to be better than me one day - I just hope you don't set up shop down the street. So in just one Summer and a half - A Pro set me lose - with both mine and his reputations on the line. I did not miss a shot at either wedding - however, one of the double exposures got the groom blinking. Not my fault - but I hated that we could not sell that image - very popular images at the time - NO Photoshoping in eyes back then and retouching such would have cost hundreds. We all have our comfort zones - it was architecture that got my first three international publications - so buildings and nature was my leading efforts as well. I understand that passion and comfort zoning - people don't really get dressed - they put on pressure suits. But that's just the fun part. Weddings and Protrat work - does much better if the photographer has a lot of charisma. Learn as much as you can - go to some weddings and watch what the pros are doing - become a wedding crasher so to speak - no one will know you were not invited - believe me - that works - if you get ran off - so what. Then stay healthy - weddings are exhausting. When you first arrive figure out where the tripod will be needed for that low light part - the KISS and either set it up in advance or leave it ready to set up and do that with as little noise as possible. Use a monipod for everything else.
Take it serious - but have fun with it as well - sounds like a comfortable opportunity of gain some experience - if the bride and groom are truly not expecting professional results. I started buying about a dozen point and shot cameras (no flashes) and gave them to people at the wedding and asked them to shot the entire roll and give the camera back to me before they left - you would be amazed at how well that worked out - got a lot of fun stuff, got a lot of KISS shots - out of a dozen camera plus mine - the dang KISS was not going to be missed. Put together a kind of flip book from those shots as well - the client loved it. Just work and work and work at it - practice with friends make them the bride and groom - that give you some confidence - go to that church and work with some friends with candles - youv'e got till September to get ready - do some practice runs - it's digital - so you are not wasting film. Take this chance to get better over the next two months - learn and gain some confidence - and then go for it. I'll bet you'll do fine - but do read up on some books on the topic - off the top of my head I can not think of a title - Oh go on line and look at some of the pros web sites you'll get a lot of good ideas there - don't worry that you might not think you can be as good - just look at what they do - and dream that you can do it as well - let this be your vision wall as to what you can do or one day do. Sounds like the bride and groom understand your beginner status - take this chance to move beyond the beginner status - that's what I did as well - with cousins weddings as my wedding gift to them - they none could afford the professional at $900 - $3000 plus prints our families were poor back in those days - they all thought I was loaded with money with my Nikon and later Hasselblad. Even I thought I was over addictive to equipment - but today that is a drop in the hat compaired to what this stuff costs today. I hope you take the tight rope and make it without the net. You don't need the net as long as there is a rope to grab. Good luck and post us your favorite results in September - and Pray and pray for the bride and groom that their marriage is successful - these days that is way greater risk than the photographer is taking. Then one day - Go Pro - and be glad you did.