Alt codes, on Microsoft machines, are only used to allow one to enter all the characters of one’s current 8-bit DOS font and one’s current 8-bit Windows font. That gives you access to a very small number of characters.
You can’t create new codes, except by editing a font to replace one character with another, but then the character will only be visible to someone who has your revised font. (For example, the font Wingdings replaces individual characters in a normal font with different glyphs, and so do other symbol fonts. But such symbol fonts do not work on the web, unless you play some tricks that do not work on every machine with every browser or unless you save your document in some format like PDF where the font glyphs are downloaded.)
There are supposed lists of Alt Code charts all over the web already, mostly created by clueless users who don’t seem to realize that the characters one gets for some Alt codes will vary according to what one’s DOS character set and one’s Windows character set are.
Any of the many sites which pretend to supply “the” Alt codes are created by ignoramuses.
Essentially you can enter into almost any computer any one of the 107,156 current graphic characters in Unicode. See http://www.unicode.org/charts/ for two tables of table of the official charts of the characters. Each character has its own unique hexadecimal code assigned, and in most forums and when creating a website one can enter any character using its code, preceded by & # x (without spaces) and followed by a semi-colon (;).
You can also use an extended Alt code method used in some Microsoft Products, notably Microsoft Office and Notepad. In these, pressing the left Alt key and typing a number from 256 to 65535 on the numeric keypad will produce the Unicode character which corresponds to a hexadecimal translation of that decimal number. Or you can download and install the free utility Quick Unicode Input Tool from http://www.cardbox.com/quick.htm to enable this extended Alt Code under any application. Note that while Quick Unicode Input Tool is running, you will only be able to use Unicode values to obtain characters, not the values of your current DOS character set.
There is no Satanic heart symbol in Unicode and so you will not be able to obtain it in this fashion.
See http://ipa4linguists.pbworks.com/ for a site intended to tell how to produce phonetic characters under Window, Macintosh, and Linux, but gives techniques which mostly work with all the Unicode characters.