Let's see the table of Basic Katakanas.
It's called 五十音表 (GOJUU-ON-HYOU fifty note table) because they includes 50 cells.

　ア　イ　ウ　エ　オ

　カ　キ　ク　ケ　コ

　サ　シ　ス　セ　ソ

　タ　チ　ツ　テ　ト

　ナ　ニ　ヌ　ネ　ノ

　ハ　ヒ　フ　ヘ　ホ

　マ　ミ　ム　メ　モ

　ヤ　　　ユ　　　ヨ

　ラ　リ　ル　レ　ロ

　ワ　　　　　　　ヲ

　ン

(*The reason why I call this as the Basic Katakanas is we need to learn some special letters as ザ, ダ, バ, パ, ォ, ョ. They are variety of the basic ones. I will describe them later.)

As I said we have 50 katakanas, but there are only 47 because some letters have been obsolated historically.

The letters are much more than roman alphabets that have 26 characters, it is quite easy to remenber Katakana repartries.
Below, I write down the list again with pronunciation in palenthesis().

　ア(a)　イ(i)　ウ(u)　エ(e)　オ(o)

　カ(ka)　キ(ki)　ク(ku)　ケ(ke)　コ(ko)

　サ(sa)　シ(shi)　ス(su)　セ(se)　ソ(so)

　タ(ta)　チ(chi)　ツ(tsu)　テ(te)　ト(to)

　ナ(na)　ニ(ni)　ヌ(nu)　ネ(ne)　ノ(no)

　ハ(ha)　ヒ(hi)　フ(fu)　ヘ(he)　ホ(ho)

　マ(ma)　ミ(mi)　ム(mu)　メ(me)　モ(mo)

　ヤ(ya)　　　　　ユ(yu)　　　　　ヨ(yo)

　ラ(ra)　リ(ri)　ル(ru)　レ(re)　ロ(ro)

　ワ(wa)　　　　　　　　　　　　　ヲ(wo)

　ン(nn)

So you find the letters are ordered in the table systematically.
Each rows are series of letters with the same root voice A, I, U, E and O.
These voices are called as BOIN (母音) in Japanese which means "Mother Tone".

BOINs are following SHIIN (子音) ("Child Tone") as K, S, T, N, H, M, Y, R, W.

So you got to learn 5 BOINs アイウエオ (AIUEO) and 10 SHIINs アカサタナハマヤラワ (AKASATANA HAMAYARAWA) then you can understand the GOJUU-ON (五十音 fifty sounds) system.

Now, Katakana is mainly used to describe the Western words in Japanese.
So let's write some Western words.

The word "America" is pronounced as "AMERIKA" in Japanese.
So the writing would be "アメリカ".

The word "Cairo" is pronounced as "KAIRO".
So the writing would be "カイロ".

The two examples above are easy to understand.
But following one is rather difficult if you are Western or English speakers.

The word "Microsoft" is proonunced as "MAIKUROSOFUTO".
So the writing would be "マイクロソフト".

Note that Japanese pronounces the Western words in their own accent.
So the words' sounds are changed firstly being easy to pronounce for Japanese, and then transformed to Katakana.

This is the irreversible process.
So the Katakana words are modified Western words.

Let's discuss about the typical problem.
In the Katakana GOJUU-ON table, you don't find L sounds.
Japanese cannot distinguish L and R by nature.
(Of cource modern Japanese CAN do it because they learns English in school :-D).

So how you can write the word Manila in Katakana?
Japanese pronounced as "MANIRA".
So the writing would be "マニラ".

But how you can write "England", "Soccer", or "Computer"?
It will be discussed next time.
Stay tune!