My girl friend and me have dated for almost 1 month and next week will be her birthday. I want to make a handmade cake for her

How can I make it and how to decorate it surprise!!!

Have you ever baked &/or decorated before? What equipment do you have?   Here are a couple of recipes for you to use:  https://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe       https://www.cakecentral.com/recipe/22469/2-icing     Here's an example of a somewhat simple design:  https://www.cakecentral.com/gallery/i/1362506/best-wishes    If you have cookie cutters and some fondant you can make decorations.  Cut out hearts, stars, circles or whatever to put on the iced cake.  https://www.cakecentral.com/gallery/i/1351608/generic-mans-birthday   or   https://www.cakecentral.com/gallery/i/1351231/generic-design-for-a-man     https://www.cakecentral.com/gallery/i/1340957/generic-mens-design   A design for a man can easily be changed by using her favorite colors.      Cut out lines of fondant and twist to put around the cake like the top of this one:  https://www.cakecentral.com/gallery/i/1351665/5-yr-olds-birthday              As you can see, there are many, many different, easy ways to decorate. 

There are hundreds of cakes posted on-line.  Just peruse them until you see ideas you like.