S100FS For Wedding Photography

My friend asked me at the last min to photograph her wedding and i have not the slightest clue how to go about it....here are the detail about the wedding location and reception afterwards

wedding will be outside in mid afternoon in her moms flower garden
reception will also be held outside in the garden mid to late afternoon
there will be large tents for the reception that may be dimmly lit if lit at all

i have never shot at a wedding before...should i invest in a good external flash and a defuser for it?.....is there any special filters you can get to add like a star or twinkle affect to certain photos....i guess what im trying to say is any tips and tricks would be greatly appricated.......oooh and i will be shooting with a fuji finepix S100fs and a S9000 for my back up
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FUJI FINEPIX S100FS,FUJI FINEPIX S9000