CATO is a new user interface, developed by the author as a response to the significant difficulties faced by scientists, engineers, and students in their usage of computer algebra (CA) systems. Their tendency to use CA systems only occasionally means that they are unfamiliar with requisite grammar and syntax these systems require. The author proposes CATO as a general user interface for many computer algebra systems (currently Maple, Mathematica, Maxima, MuPAD, MuPAD inside MATLAB, and Yacas). The interaction CATO enables is independent of the CA system used. CATO implements various concepts for the guided use of computer algebra. Commands with more than one parameter have a two-dimensional input structure; the parameters being explained, and the right kind of separators and brackets are set by CATO. Selecting and adjusting options for commands always follow a similar process. For each command, commonly used paraphrases are accessible under "Help". CATO is currently based on German, but the proposed concept is easily extended to other languages. CATO simplifies access to a selection of commands by combining them with discrete packages. Commands are often contained in more than one package, as they may be linked to more than one concept. The "definition of a vector" command, for instance, is contained in both the "definitions" and the "linear algebra" packages. Users are able to access their most recently used commands from in-built lists that store the history of command invocations. Users are also able to define such lists as new packages, and can export and import these as their own packages.