You have been given at least partly-correct information by most the other answerers here.
For starters, it depends on precisely what you mean by "hearts."
In chatrooms, forums, texting and whatnot, the convention for indicating a heart is what others here have told you: The <3 character combination.
Some forum/chat software will see those two characters used like that and will convert them, in the actual posting, to a graphical heart. But even if it is not so automatically converted, most people know what those two characters mean.
However, there is a heart "extended" character in many Windows font sets. One need merely open the character map utility, find it in whatever font one is using (or use the one in the "Wingdings" font) and copy the extended charcter to the clipboard, then paste it into whatever one is typing.
One may also, if one knows how to use the [Alt] key and the numerica keypad (and if one knows the precise code) insert the heart character (if one is using a font which has it) that way.
Hope that helps.