I am an absolute beginner with all this and am wanting to do a cake for the FIL . I had an allotment one in mind but the question is where do I start??? Marzipan vegetables I could manage, but what about a fence or shed? Do you think thats pushing it for a beginner? And silly question but what do I make it all out of??
An allotment is a garden that you rent from the council, at least it is in England as far as I know 
Maybe something like this?: http://www.flickr.com/photos/33785888@N02/3147783064/
Looks like she used Cadbury fingers for the fence which would be easy enough if you are a beginner, and the shed looks like a square sponge cake with chocolate sprinkles stuck to the side, on top of the icing. You can do the vegetables and path out of marzipan, or sugarpaste.
I don't know what is available in the UK, but here we have a lot of candy molds and I noticed one that was a mold for tiny little vegetables and a shovel (spade) and rake-so cute! Maybe you could find it or buy online and save a lot of work. I also think crushing up those thin chocolate cookies would make great soil for the garden.
HTH
the fence could be piped with royal icing (use a template of a picket fence panel. Outline it with medium consistency royal icing or color flow. Thin some out and flow in and allow to dry completely. To reinforce it, once dried, turn over and outline and flow in on the back side of it). The shed could be made with royal icing as well...or even gingerbread cookies cut into shape then decorated and assembled with royal icing. Bean poles, tomato cages could be made with the help of spaghetti for structure....
possibilities are endless!
Hello cupcake (the book) has a garden themed cupcake set that might help you. They use candies of all sorts to make the veggies, and her books are aimed towards making things easy. The link below has a full list of candies (i think) so maybe that will help 
good luck, post pics when you are done!!
http://www.womansday.com/Recipes/Garden-Party-Cupcakes
Gosh, thank you for all the helpful replies.
I have the cake done - now for the tricky bit..... am still waiting for the cupcake book to arrive - hopefully tomorrow !!
Think I shall go for the chocolate fingers as a fence - how do I make them stick on??
I have been practicing my mini marzipan carrots etc - do you think I need to glaze them and again how do I stick them on ?? The ones I did have gone hard - is this normal or is there a trick to keeping them soft?
Have yet to decide about the garden shed -keep waiting for the book in the hope that will 'save' me !! Oh and I shall be doing crushed up biscuits as soil so thank you again. You have all been so helpful.
Bit nervous about doing the finished cake but I am sure(she says) all will be well.
all4cake posted the link to my "Fruits of His Labours" cake. I made the shed using the template I just uploaded to CC.
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1768583&done=1 I scaled it on a photocopier to get the size I wanted.
I textured the roof with a new pan scourer and the walls using a cocktail stick. Just allow time for the different pieces to dry before trying to stick them together. Cabbages were made using a rose petal cutter; cauliflowers using a more rounded leaf shape and a ball of textured white paste. All elements were from modelling paste (CMC added to sugarpaste) except those described. If I had added a fence I would have used Doug's template and attached it with either edible glue or softened sugarpaste. I had great fun doing this cake although doing all the elements took some time so that is why I did a topper so it could be done before I made the cake. I'm a newbie too so if I can so can you. 
PM me if I can be of any more help.
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