Summarize the following:
Whatever the occasion is, you are likely throwing the party for a person or a group of people. Are they children? Adults? Are they close friends, fellow classmates, or co-workers? Knowing who your guests will be will give you a better idea as to what sorts of themes, colors, and decorations you should use. For example:  If your guests are mostly children, you'd want to use lots of bright colors, balloons, and streamers. If your guests are older, you might want to limit the colors and use simpler decorations for something more elegant and/or sophisticated. Is this a dinner party? If so, you'd want to focus more on the table itself, as this is where your guests will spend most of their time. Get some nice plates, napkins, tablecloths, centerpieces, and so forth. Focus less on extras, such as balloons and paper cutouts. This will also help you decide what sorts of colors and decorations to use. It will help make the party more personal and special. For example:  If it's a baby shower, consider whether it is going to be a boy or girl. Use lots of blues for boys, and lots of pinks for girls. don't use too much shades. If it is a birthday or Quinceanera, use the birthday girl's favorite colors. Try to stick with one main color, and white as a secondary color. If it is graduation party, consider using the school's colors. If it is for a holiday (New Year's, Halloween, Christmas, etc), use colors associated with that holiday. For example, if it is a Halloween party, you might want to use orange and black. If it is a more formal Halloween party, use white and black. If it is a wedding party, consider the wedding's theme colors. These will usually match the cake, wedding bouquet, and the bridesmaids' dresses. The theme will be most likely something that the person(s) you are throwing the party for is interested in. If you don't know what the person is interested in, then choose something that is currently popular (such as rustic, vintage, or antique). Here are some ideas to get you started:  Consider the person's interests. Do they like fish? Monsters? Ponies? Choose decorations that match these interests. Do you and your friends have a favorite book or movie? Base your party off of that. If the book or movie is new, you might find the right decorations in your local party shop. Try other general themes, such as rustic (lots of burlap and galvanized steel) or antique (lots of lace). Take your guests away to another time and/or place. Try a Tiki, Luau, or Paris themed party. You can also try another time era, such as the 1920s, 1950s, or the 1970s. Take a look at the calendar, and note any upcoming holidays. If you feel that the holiday might get in the way of your party, then take note of the season: fall, winter, spring, or summer.  If your employee-appreciation party is set during October, you might want to give it a fall or Halloween theme. Use lots of orange, pumpkins, and bats for your decorations. If your birthday is set during December, you might want to stay away from a Christmas theme, but you could consider a winter theme. Use lots of blues, whites, silvers, and snowflakes for your decorations.

Summary:
Decide who the party is for. Decide what the occasion is. Chose a theme. Use the upcoming holiday or current season as inspiration if you are having troubles coming up with a theme.