Put your text into the 'text' box: click the letters, paste text copied from somewhere else, type into the box, whatever. Then click somewhere outside that box to get your enumeration.
Please note that the calculator only recognizes characters you can see in the buttons. It won't recognize Greek characters when your system is not 'Greek'. Be cautious of that when checking the enumeration.
Some of my non-Romance choices probably look weird, such as using 'C' for the Hebrew Tzaddi, and you may well disagree with those choices. I had to do something to provide for cases such as Hebrew "final" characters and the Greek Theta (Θ) or Digamma. (As a note, I base my Hebrew choices on this modified Michigan-Clairmont transliteration scheme.) The calculator only recognizes the character you see. If you use the wrong case, the calculator will ignore that letter.
Spaces and punctuation are always ignored in gematria, and some letters are ignored in some systems. Ignored characters won't affect the numeric total, though they will appear in the converted text.
I haven't provided any information about the various systems. All of them have documentation elsewhere on the Web, and all of them either are now or were once in use. If you know what a system is and want to use it, that's what it's there for. If you don't know what a system is, you'll need to research that system somewhere else.
...Actually, there is one system I will explain: "Cute Purple Dinosaur", otherwise known as "Barney". This is an old internet joke, proof that Barney is the Antichrist. In this system, the only letters that have values are the ones that can be used as Roman digits: I, V, X, etc. The others are ignored. Now type in "CUTE PURPLE DINOSAUR", and guess what you get?