Although I highly recommend learning Chinese characters from the start, many learners are dependent on pinyin - at least at the beginning of their studies. So, with that in mind, I present the basic tone mark rules for writing in pinyin.
When writing in pinyin, the placement of the tone mark is determined according to the following rules;
1. A consonant can never take a tone mark so if there is only one vowel, that vowel takes the tone mark.
2. 'A' and 'e' trump all other vowels and always take the tone marks. Since there are no words which contain 'a' and 'e', remember this rule and you should be fine.
3. If the vowel pair is 'ui', the 'i' takes the tone mark.
One of the easiest ways to remember the rules is by this crazy little pattern; alphabetical order dictates which letter gets the tone mark. (if it’s “biāo” then the “a” is alphabetically first, so give it the tone mark). The only exception I’ve found is the “iu” combination. In this case the “u” gets the mark even though it’s alphabetically after “i”
*Note: when writing a tone mark over an 'i', bear in mind that the dot on the top of the 'i' should be replaced by the tone mark, you don't need to write both.