Pick a theme, any theme. Then pick a color.
So,we will pretend you picked princess theme, and the color pink.
Now, you do everything in that color and that theme, according to your budget, and skill level. Check the local craft, discount, and party stores to see what they have available, before you start spending money.
Find pink curly ribbon for the balloons, rent a helium canister, and fill the balloons. Tie them off, and tie a bow next to the balloon, and leave a long string to tie to something else. Or, go to the local party store, or to the local bargain store. Buy the balloon, and they fill it for free. At the local bargain store, it is a buck to buy one. You'll be wanting one with a picture of a princess.
The party store, or your local craft store will have coordinated invitations, plates, cups, thank you notes, even table cloths.
Contact the local bakery, or do it yourself for the cake. They can print a princess design, , design one by hand, or even make a princess glass slipper for the top, depending on where you go, and how much money you have to spend. Check out sites like Martha Stewart, for cake ideas if you are doing your own. Check out the local cake decorating or craft store for ideas, custom bake pans, and decorating kits.
Buy presents that fit the theme, and wrap in the chosen color and theme. Tell the guests what the color and theme is, but they may not want to go along. A good hint is when they get the the invitation with a princess on it.
Make the invite say what you are doing. "Pricess Cinderella and Price charming invite you to the birthday party they are giving for Janey on her 2nd birthday."
Cute idea is to buy some cheap tiarras for the guests, maybe some cheap boas, so every one can dress like a princess.
You can do this with any theme you choose. Teddy bears and blue. Barney and his own color of purple. Big Bird and yellow.
Hope this helps. Happy birthday to your 2 year old!