How Do I Get Over The Fence In The Beginning Of The Game Of HM: Tree Of Tranquility?

What do you mean by the fence? Supposing you mean the fence at the seed store (I can't remember the name right now), you don't. You talk to Anissa's dad and he will give you the assignment. You'll appear in the garden and have to do the work. You may have to talk to him twice before you show up in the garden. It's been a while and I've forgotten.
If you mean the fence in town, surrounding the mayor's house, again, you don't. You can only get past that fence later in the game. If you meant something else, please reply. Hope I could help.