The wide variety of 1st birthday party themes for girls themselves can leave you royally perplexed. Some options are a princess theme, Sesame Street theme, beach party theme, Barnyard theme, fairy theme, Barney theme, Luau, Winnie the Pooh theme, zoo theme, Barbie theme, carnival theme, or beach theme.
If your baby has a favorite toy or musical group (like the wiggles), you could base it on that. These themes are very popular, and so, there is plenty of partyware (paper products, decorations, favors, etc.) available. However, theme decorations are fairly easy to make too.
Some More Ideas
For homemade party invitations, cut out a large number 1 from a brightly colored sheet of card paper. Used glitter/calligraphy pens to list the details of the party on one side of the paper. On the other side, write "Look who's turning 1!", and paste a picture of your baby girl. Write her name below the picture. And add any other words you may wish to.
A few other 1st birthday party ideas for girls are takeaways such as, tiaras (especially if it is a princess theme), blowing bubbles, finger puppets, or picture books. As for location, you can have it at your home, a friend's or relative's home, a restaurant or a playground, though a familiar location will be the best.
Another pointer for a successful first birthday party is to keep it short, as the younger kids are sure to get tired and cranky. You could even have a morning party, which works well with young children.