There are special cake toppers you can place on the cake that have slots to arrainge the flowers in they also press down into the cake. They also have special flower tips to place a few flowers inside. They are sold at florist shops or some of the decorating websites also have them. You don't want to push the flowers directly into the cake itself.
There are many flowers that can be used on a cake there are others that are poisous so make sure which ones they are there is a list on this website. Also you can have a florist meet you at the venue and they will place the flowers on the cake for you depending on how many are to be placed on it. Of corse that costs extra but it depends on if you are good at arrainging flowers. HTH
Yeah, I arrived at a venue once where the florist was suppose to have the flower arrangements ready for me to place on a cake. I was never told what the flower was, but it was an orange calla lily - very poisionous. The florist said the bride never said anything to her but she handed me a few of them and said I could use them. I was pissed! I had to go back to my kitchen, gather up floral tape and pastry bands. I cut the pastry bands so their was a barrier between the flowers and the cake, so yeah it didn't look as nice with the plastic on it (I couldn't fully hide it), but I was not about to let someone get sick or die due to the flower choice of the bride placed on my cake!
Just some advice, NEVER place stems or let someone else place stems directly into your cake - YUCK! I always try and put some sort of barrier between my flowers and the cake.
Jen 
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