Decide on the layout of the poster you want to make. The layout is basically the size and dimensions of the poster. If you are eventually going to print your poster onto paper, consider image resolution and paper size. If your poster is for online and digital purposes only the resolution shouldn't be an issue, and size can be adjusted to fit different screen sizes. Some common poster sizes are 11 x 14 inches and 14 x 17 inches. See the Resources section for some inspiring poster designs.
Decide on the content of your poster. After you know the layout of your poster you'll want to come up with some text and images to use. Search the Internet for images that you might want to use and download or bookmark them. There are many free stock photo sites, such as stock.xchng, from which you can search and download photos easily. Write out the text you want to use on your poster, don't worry about the location or font of the text yet.
Find the fonts you want to use for your online poster by searching font sites. There are a number of free font sites on the web, such as 1001 Free Fonts, that offer a wide variety of fonts for every occasion. There are also more professional fonts that you can purchase online from sites like Fonts.com. You will probably want to use several fonts for your poster, making sure to differentiate between headlines, subheads, and body text.
Download other helpful online poster resources to help you design your poster. There are a number of online tools you can use for your poster such as brushes and patterns, available at Brusheezy, that will greatly aid you in designing your poster.
Design a first draft of your poster using all the fonts, images and other tools you've acquired online.
Show your first trial to some friends and get critical feedback. An important part of all artistic creations is to get feedback and make revisions. Don't be afraid to drastically alter you poster from its original form. As long as what you finish with is satisfying it doesn't matter how many edits and revisions it takes to get there.