Password security is an important issue - you should not share your passwords with others, or leave passwords where others can find them (i.e. don't write your passwords on sticky notes and put them on your desk, don't store them on your phone, etc.)
Fortunately, there are many good options when it comes to password management.
To manage your McDaniel College password, use the ReACT system.
You should also follow each site's guidelines for a "strong" password, which generally means combinations of upper- and lower-case letters, numbers, and symbols as part of your password. Some sites, particularly those that deal with financial information, also require what is called "two-factor" authentication, which sends a text message to your phone with a code to use in addition to your password.