Reading text files to Matlab variables is one of the most commons tasks. One of the many, many functions is my readfile function, which is based on the internal fileread function (where my function also tries to account for the encoding, which is probably irrelevant in your case).
You can indeed use fopen, but you do actually need to use the functions with their intended use: fopen will generate a file identifier, which is a kind of handle that points to a specific file. You need it if you want to use fread or fprintf, two functions I would highly encourge you check out. Also note that every function documention page has a footer that contains links to several related functions that could be usefull.