For me, the cake is the easy part. But I don't have the patience or skill to make thousands of fondant flowers or smooth buttercream perfectly. That's why I always choose foolproof, classic decorations like satin ribbon (be sure to cover the back with clear plastic tape so the fabric doesn't become greasy, and secure with a pearl-headed pin) and fresh flowers. Covering frosting with shredded coconut or white chocolate curls is another easy way to hide imperfections. I've also seen a gorgeous wedding cake covered in rainbow sprinkles. This frosting that looks like a cloud is simple but looks dreamy. Another rustic style I really like that favors the lazy froster is having a "naked" cake, with filling and frosting on the top, but little to no frosting on the sides. Choose decoration you feel comfortable doing and remember that simple is usually better.
You should use a recipe specifically designed to be a wedding cake as it will ensure that it is sturdy enough and that it makes the right amount of batter and icing. For a three-tier cake, you need to make three cakes of 12", 9" and 6", as well as a massive amount of icing. Most three-tier cake recipes have a step-by-step schedule for when you will need to bake and assemble each part.