A. The following words should always be capitalised:
the first word of every sentence
the personal pronoun ‘I’
proper nouns (individuals' names, place names, planets, mountain ranges, oceans, languages, days of the week, months of the year, brand names) - remember that Earth should be capitalised when referring to the planet, but lower case 'earth' when referring to the soil.
Roman numerals, e.g. XIV, IV, MCLXVI
Geological periods (Precambrian, Archaean, etc.)
N.B. we do not use a capital letter after a colon, unless the word would need a capital letter in any context, e.g. a proper name - Precambrian, Earth.
B. Specific, allocated names should be capitalised when things or creatures are being referred to by that specific, unique name, otherwise use lower case. This can be complicated so let's look at examples:
the Mid-Atlantic Ridge, but the ridge
the Great Rift Valley, but the rift valley or rift valleys
the Somali subplate, but the subplate or subplates
the Atlantic Ocean, but the ocean or oceans
Professor Brainstorm, but the professor
South America, but the southern American states
C. With scientific taxonomic classifications, everything above a species is capitalised: Tip: a quick google will often tell you whether you're dealing with a genus or a species.
Homo sapiens (Homo, which is the genus, is capitalised; sapiens, which is the species, is not capitalised)
the phylum Arthropoda (phylum, which is the species, is not capitalised; Arthropoda, which is the genus, is capitalised)