Decorations for wedding and reception? clueless!?

I am so bad with this sort of stuff...were having our wedding at a church and the reception at the fire hall banquet hall area. all the tables, napkins, etc are white (not my choice, its from catering) the colors for the wedding party is dark blue. how should i get flowers? where should i put them? do you put them up at the actual wedding too? on the tables at the reception? since its at a fire hall i don't really know where i would go to get decorations for the tables and such. I feel so lost i just want it to look nice. any ideas of budget things? im the first of my friends/family to get married so im kind of at a lost (also have been to few weddings)