There are a few more things that you need to know about how katakana are used:

Small tsu
A small tsu in katakana is used in exactly the same way as in hiragana. A big tsu is pronounced, er, tsu, whereas a small tsu acts as a sort of double consonant.

hato (pigeon)

hatto (hat, cap)

With the addition of the small tsu just in front of it, the to has its initial t-sound "doubled"; instead of hato we get something more like hat-to.

Long vowel sounds
Katakana uses a character after a syllable to lengthen the vowel sound. To pronounce this correctly, just hold the vowel for an extra beat.


biru - building


biiru - beer

Foreign words
Probably the most important use of katakana is for writing words of foreign origin. Most foreign words are quite easy to represent using Japanese sounds, but there are plenty of foreign sounds which have no direct equivalent in Japanese. Problems can therefore arise when you try to write them in katakana. Let's look at some examples:

amerika - America


ko-hi - coffee


sakka - soccer


chokore-to - chocolate


bideo - video


biniiru - vinyl

Some points, then, in no particular order:

ra / la
The katakana / / / / are used to represent both ra / ri / ru / re / ro and also la / li / lu / le / lo.

fa / fi / fu / fe / fo
The only Japanese syllable to start with an English f-sound is fu; there's no single character for any of fa, fi, fe or fo. You can expect to see the katakana for ha, hi, he and ho used instead (as in ko-hi, above) but you will also see, in some katakana words, fu followed by a small vowel:


famirii maato - Family Mart (convenience store chain)


fenshingu - fencing

Which convention is used depends on the word in question - I'm not aware of any general rule, so I tend to learn them case-by-case.

ba / va
The consonant v doesn't exist in Japanese - instead, the katakana for ba / bi / bu / be / bo are used. Hence bideo (video) above. One interesting alternative, though, which seems to be becoming more common these days, is the use of a katakana u plus two dashes in the top-right corner (like a voiced ga or da, for instance) to represent the (non-Japanese) syllable vu:


vo-karisuto - vocalist

Now, from a linguistic point of view, this is nonsense - there's no such thing as a voiced or unvoiced vowel. So don't think of it that way - just remember that u plus two dashes is vu. As the above example suggests, this vu syllable can be combined with a small vowel to produce va / vi / ve / vo - but in practice, ba / bi / bu / be / bo are much more common.

w-sounds and y-sounds
The only Japanese syllables starting with a w-sound are wa and wo. Even then, the katakana for wo isn't used very often. Instead you can expect to see a katakana u followed by a small i, e or o. Similarly, a ye sound tends to be written with a katakana i followed by a small e:


uettosuutsu - wetsuit


uo-kuman - Walkman (TM)


ieroo - yellow

th
There's no th-sound in Japanese. Z-sounds or s-sounds are typically used instead. So if you're looking for "The Beach" in your local video store, it'll be filed under "za biichi". (That was a public service announcement for all you Leonardo DiCaprio fans out there.) And the name Gareth, for example, is spelled garesu in katakana.

open syllables
Most syllables in Japanese are open; that is, they end in a vowel sound. In fact, the only closed syllable is n. This means that, unless a word ends in a vowel or an n, its katakana version is going to have an extra vowel on the end. My name, Chris, is one syllable in English, but three syllables in Japanese: kurisu. Similarly, Paul becomes po-ru; again, three syllables and with an extraneous -u sound on the end.