Always start from the beginning. Does your ruler have some space between the end of the ruler and the first long mark? This line marks zero. A space before the zero mark allows a decorative end to the ruler but it also insures that you'll always have an accurate zero starting point if the end of the ruler begins to wear down. If you need to use the smallest measure on a ruler, you'll get more accurate readings if you use a wood ruler with some space before the zero point or a metal ruler that doesn't wear with use.
Look at your ruler. There will be a set of numbers, each of which is next to a long line. Most rulers have 12 numbers marking inches. Some are longer. Between each number, there will be a group of shorter lines of varying lengths. Each inch will be divided in half by the next longest line and the haves into two quarters by the two next longest lines. The shortest lines between all the others mark the smallest measurements on most rulers--sixteenths of an inch. There will be eight of the shortest lines--one between each of the longer lines. The lines between the numbers divide each inch into sixteen parts. Note that there are eight sixteenths in a half inch, four sixteenths in a quarter and two sixteenths in an eighth.
Read your ruler by noting the inch, then adding the number of sixteenths of the inch. Or, more simply, note the number of half (eight sixteenths) or quarter (four sixteenths) of an inch and add any extra sixteenths to get a total number of sixteenths. It works the other way, too. If you end up with an even number of sixteenths, you can reduce the fraction to end up with eighths (two, six, 10 or 14 sixteenths) quarters (four, eight or 12 sixteenths) or half (eight sixteenths) inches.
Be careful when reading sixteenths. Most measurements you make won't need to be this exact but if you're doing carpentry, building furniture or measuring mechanical parts, you'll need the precision of sixteenths. Hardware and drill bits are measured in sixteenths--and a mistake on a bookshelf or pipe fitting can waste time and money. If you plan on doing a lot of carpentry, invest in a good metal ruler so you don't have to waste time squinting.