Katakana is the Japanese script which is used to write loanwords. These words are not of Japanese origin or in other words, a Japanese term does not exist for them. Katakana consists of 5 vowels (ア、イ、ウ、エ、オ); 40 consonants and（ヲ) a katakana of the particle を(in hiragana), but it is almost never used. The consonants are formed by attaching a vowel sound to a consonant sound (カ、キ、ク、ケ、コ……). A single consonant forms a mora. However, there is one exception to this, i.e. ン. This consonant never starts a word.
When the ‘dakuten’ or ‘ten ten’, a symbol looking like [ \\ ] is add to the top right of the consonants producing ‘k’, ‘s’, ‘ts’ and ‘h’ sounds, we get the ‘g’, ‘z’, ‘d’ and ‘b’ sounds respectively. This diacritic changes voiceless consonants into voiced consonants.
NOTE when dakuten is added to シ ‘shi’ and チ ‘chi’ the sounds produced is ‘ji’ in both cases but NOT ‘zi’ or ‘di’. ヅ is pronounced as ‘zu’ and not ‘du’.
Handakuten is another diacritic which is put on the top right of the ‘h’ consonants to get the respective ‘p’ sounds. It looks like a small cirle [ 。].
When ヤ ‘ya’, ユ ‘yu’ or ヨ ‘yo’ is added to the right of the ‘i’ ending consonants the following sounds are produced. (This palatalization of ‘i’ ending consonants with ‘ya’, yu’ or ‘yo’ is called youon)
NOTE that the ヤ ‘ya’, ユ ‘yu’ or ヨ ‘yo’ should appear smaller than the ‘i’ ending consonant it is added to.
The consonant ‘y’ in English is considered a semi vowel or a glide. In Japanese, the ヤ ‘ya’, ユ ‘yu’ or ヨ ‘yo’ are adjoined with the ‘i’ ending consonants （キ、シ、チ….) to create a glide.