At Michaels they sell these little plastic "vases" in the floral departmet (they are pointed and hold water with a stopper in them so it doesn't leak) the flower fit right into the top of them. You would push them into the cake like you would a support rod. Also make sure that the flowers you are using are organic. No pesticides or anything like that. If they touch your cake that can come off onto your icing and make you sick. Hope that helps.
Here is something of interest for you...a link to a list of non-toxic & edible flowers:
http://whatscookingamerica.net/EdibleFlowers/EdibleFlowersMain.htm
Good Luck! 
I just did this for the first time yesterday. I looked for those flower spikes that wilton sell, but didnt find them at the store-I will have to look for the ones mrboop talked about. Anyway I asked here on the forum for alternatives and someone mentioned drinking straws. I just covered the ends of my stems with plastic wrap them placed them into the straws and stuck it into the cake. It worked pretty well, but i couldnt really add water to them (I just wet the plastic wrap a little) so I placed them on at the last minute. I guess I can't add the photo as an attachment because it is too big for the forum but it is located under the "green" category with orange lillies on top.
Tat--
You can put a bouquet on a cake but first place wax paper that is cut the same size as the top of the cake, dab of frosting to hold bouquet if it is in styrofoam. Lots of times, I put my wax paper on top with a dab of frosting then put a mound of frosting on that and cut/place flowers in it. Arrange to hide frosting and paper. That's the way that most of mine are done. As for the daisies on top of the bottom tier, just wrap stems in plastic wrap or foil and push into cake. Hope that helps--Pam
Thanks Mac! I think I'm going to go with your suggestion. I think the top arrangement will be in styrofoam so the wax paper idea sounds like a winner to me!!! LOL...I really appreciate all of the suggestions..Using fresh flowers will certainly be easier now that I have this knowledge!! Thanks Everyone!!!!!! 
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