When considering the use of material or ideas that you did not create, both legal and academic considerations apply. Legally, you must be careful to comply with applicable copyright laws. Academically, you are expected to acknowledge ideas and work that were created by someone else, even when you are using them legally.
In general, any content you find on the web will be subject to copyright, and may not be appropriated without permission. This includes both text and images. There are a few exceptions: materials published prior to 1923 are in the public domain, as are most creations of the United States government regardless of when they were published. Certain sites allow use of their materials according to a specific license agreement. For example, linkware refers to content (usually images) that may be used provided that you include a link back to the site of origin. In limited cases, the doctrine of fair use may protect the use of excerpted text without permission of the copyright holder, but this is a gray area and does not generally apply to images.
Unlike specific word and images, copyright law does not protect ideas or techniques. (A particular technique may theoretically be covered by patent law, although you are unlikely to run across this in a web page script.) Thus you may legally copy a CSS method used to achieve a particular layout, or the method used by a script you like, to adapt for your own purposes. (Copying the script or CSS verbatim from the source may not be ok -- thus the word "adapt".) Although appropriation of ideas is legal, in an academic community it is important to acknowledge their source. For your projects in CSC 105, you will generally do this in the project documentation.
For grading purposes, you will be assessed in terms of how much value you have added to a borrowed technique. If you simply take someone's existing design without change and merely fill in your own content, that represents a small contribution and generally will not merit a high grade. Using a software package that builds a site design for you is similarly discouraged because you need to work with the underlying techniques in order to learn them. Pulling together concepts from several sources into a unified whole, or significantly modifying an existing idea so that it accomplishes something new, are actions that show web design mastery and will be correspondingly rewarded when your work is evaluated.
Please note that the material above is intended as a general introduction to matters of copyright and attribution, and is not intended as specific legal advice!