It's easier to adjust than to create from scratch. Find an open source project you admire or use a lot, and try to tackle some minor task. Or, search a source code site (e.g. github) for a project with a relatively small codebase, and just play around with the code, to see how changing something in the code brings about a change in behaviour or output. You could begin by changing hard-coded data first (such as strings, or initialization values), then move on to change actual logic.
Make it a point to explore more than one language -- hopefully at least three. It's easier to work with higher level languages; I recommend that your list of languages to try includes Ruby or Python.