There are many ways. Gather data through field observations, conduct interviews, counting and measuring surveys, maps, photographs, satellite imagery and censuses.
One of the most important skills needed by a geographer is the ability to be observant. By being observant, a person is highly aware of what is happening around him or her. Observation relies on the use of the five human senses: sight, sound, smell, taste and touch.
To assist in answering questions about the world, geographers often obtain information from other people. Since there are many people with a special understanding of a particular topic, their knowledge and opinions can be important resources.
Two popular types of surveys used in geography are counting and measuring surveys. Counting surveys involve the counting of objects such as cars, animals and people. Counting surveys can be used, for example, to determine traffic levels on particular roads at certain times of the day and week. This information is often useful for governments, for example, that may want to assess whether the road will be able to withstand the demands placed on it.
Measuring surveys are also simple to conduct. They do, however, require measuring equipment such as tape measures to provide accurate results. There are many different properties which can be measured in surveys. These include length, height, size, temperature, speed, weight and volume. A measuring survey could be used to show how much a tree is growing, by measuring its height throughout the year.
For further reading, see Skwirk.com's Geography - Gathering Information.