I am very new to cakes and I am wanting to try to do 3D/cake sculpture, what is the best kind of cake to make that will be the easiest to carve?

Something firm, like a pound cake is the best. But you can firm up just about any recipe!

how can I firm up any recipe???

I use a doctored cake mix. Adda box of DH cake mix a box of pudding mix in whatever flavor you want, 4 eggs, 1/3 cup of oil, and a cup of water.
HTH!