Yes, you can begin a sentence with 'then'. It is often used when speaking about things that happen in sequence or something that logically follows something else. "I rented a car. Then I drove all the way to Italy from France." "You should take swimming lessons. Then you would feel safer playing in the ocean waves."

As Keyra pointed out, "Then" is a good way to start a sentence referring to the previous sentence. As such, it would not make much sense to start a paragraph with "then", but in a sequence as the one you provided, it's perfectly logical and correct. Note that you can change the punctuation and add "that" if you want to use "so" to express the same thing: You should buy a computer, so that you would be able to send e-mails. Note the difference between "therefore" and "because. "Because" is used BEFORE the explanation/reasoning and AFTER the consequence. "Therefore" is used AFTER the explanation/reasoning and BEFORE the consequence. You should buy a computer because you need to be able to send e-mails. You need to send e-mails. Therefore you should buy a computer.