The goal of standard language is to allow efficient communication. To avoid that individuals need to first agree upon a protocol every time they want to talk to each other, standard language was invented with standard rules. If you believe it is OK to deviate from those rules and invent your own punctuation, words, or even entire language, I have only one thing to say to you:
Punctuation helps to make written text easier to parse. Standard punctuation rules make every text equally easy to parse with no surprises, as long as every text follows those same rules. A text that deviates from the rules is harder to parse, hence harder to read and harder to understand.
Next to clueless people who make a wild and random guess every time they have to start a new sentence, there are also the more curious examples of people who strictly abide by their own invented syntax. A nice example is a mail I received where someone never used capitals, and never typed spaces after punctuation marks. The result was that whenever he started a sentence with “it,” I believed he was pointing to some Italian website like ‘link.it’ or ‘amazon.it’. To add insult to injury, my mail program thought alike and made the unintended links clickable. Confusion is not a basis for good communication.
Therefore, a simple list of very basic punctuation rules that apply to the majority of languages written in Roman script.
The long version with bad examples:
In case you start a sentence with a number, do not capitalise the first word that comes after the number. Consider the number capitalised. Of course, it is better to avoid starting a sentence with a number altogether, or to write that number in full. Compare the next examples. 2 ways to start a sentence with a number. Two ways to start a sentence with a number.
The only other words in a sentence that should get capital letters are names. Some languages may have additional rules for capitalisation, for instance in German every noun is capitalised.
Note that in accordance with the item below, no capitalised word must follow after a comma, colon, semicolon, dash, or other punctuation mark that does not indicate the definitive end of the previous sentence, except if the word is a name. This means the following is correct. WARNING: do not capitalise the word “do” here.
A side note about the ellipsis: although it looks like a sequence of 3 periods, it actually is a punctuation mark on its own. It is OK to simply type 3 dots (not 2 or 4), but many input methods allow to directly enter the ‘horizontal ellipsis’ Unicode character. For instance on a Mac U.S. keyboard layout, it can be entered by holding down the alt key and typing a semicolon ‘;’.
Regarding abbreviations at the end of a sentence: the last period of the abbreviation also serves as the period ending the sentence. Never try to use 2 periods, otherwise your punctuation will be F.U.B.A.R.
(1): About spaces before question marks and exclamation points: some style guides, especially in French, will suggest to put a space before these two symbols, as well as colons and semicolons. However, in most other languages the rule is to omit the preceding space. A problem with adding a space is also that in typed text that could be displayed inside a variable width text field, the space could cause the punctuation mark to drop to the next line.
There are some region-dependent rules as well about whether a comma, full stop (period), or other punctuation mark ending a sentence should be inside or outside a quoted phrase. Look them up if you want to be sure, but obviously this is only a detail. The above rules are far more important.