A first birthday party is really for the adults in attendance. The "celebrant" really isn't going to care one whit. With that in mind, plan your party to impress the adults (and the older children, too).

By all means, bake the cake yourself, but feel free to use a box mix if you've never done scratch before. Box mixes are pretty decent nowadays. Most people can't tell the difference anyway and don't care, as long as they get their sugar fix. (Somewhere on these message boards were tips to make a box mix taste like homemade, but I don't remember where. Unless your guests are cake connoisseurs, I wouldn't bother...not with a baby underfoot.)

Some theme ideas:
Balls 'n' Balloons
Spring Flowers
Bunnies
Farm/Baby Animals
Baby Toys--trucks, blocks, etc.

Simpler is better when it comes to a one-year-old. Less stress and more fun for you, too. Enjoy!