Russian punctuation is very complex and difficult even for native Russian speakers. Many of those graduated from the high school hardly can put all the commas and dashes in the right place. However, if you are dealing with business letters, you need to understand at least the basic rules.

Comma

1. After the name of the one you address to: Оля, принеси мне чай. Ola, bring me some tea.

2. When enumerating something or describing something: У нас были белые, синие и красные карандаши. We had white, blue, and red pencils. Mind that you should not put a comma before teh last item if there is an "и" copulative conjunction.

3. After a parenthetic word: Однако, он бросил эту затею. However, he gave up the idea.

4. Between two or more simple sentences consisting of a composite sentence if there is a conjuction between them. Нам удалось найти ключ, но мы не смогли открыть ворота. We managed to find the key but we failed to open the gate.

5. Before and after the dangling participle: Он шел вдоль дороги, напевая какую-то мелодию. He walked along the road, singing some song.

6. After an interjection: А, это ты... Oh, it's you...

Colon

1. After a summarizing word: В коробке было много разных вещей: книги, ручки и даже глобус. There were lots of things in the box: books, pens, and even a globe.

2. In asyndetic composite sentences in some cases: Он с трудом досидел до конца урока: ему было скучно. He hardly managed to sit until the end of the class: he was so bored.

Semicolon

1. In asyndetic composite sentences in case it's too long and has other conjunctions: Его выгнали из университета задолго до этих ужасных, непонятных, даже нелепых событий; наличие факта обмана даже не пугало его, хотя должно было бы насторожить. He got flunked from the university long before these horrible, odd, even absurd events happened; the fact of lies didn't scary him, though it should had put him on his guard.

2. After the last word of the line in a list:
Ex. Нужно купить:
1) Молоко; 2
) Яблоки;
3) Журнал.

To buy:
1) Milk;
2) Apples;
3) A magazine.

Dash

1. A dash should be put between the main members of a sentence (a noun or a numeral in the subjective case or an infinitive) when there is no link-verbs.
Мой брат - инженер. My brother is an engineer.

These are just some basic rules you should follow when writing in Russian. There are a lot of other rules and principles which you can learn from the proper textbooks. Good luck!