The simplest way to begin to track your portions is to buy and use a simple measuring cup. Many foods such as cereal and pasta are measured by the 1/2 cup or cup. Fluid ounces, such as a beverages, can also be measured with a measuring cup. Some foods such as peanut butter, pancake syrup, or jam are measured by the tablespoon. A few foods, such as stick margarine, can be difficult to measure with a spoon; a serving of margarine, for example, can be measured by using the "ruler" provided on the the product's packaging.
Other foods will list a serving as "x number of slices" or "x pieces" such a deli meat or candy, where you can simply count out the number of pieces that make a serving.
Some foods list a weight, such as an ounce, as a serving, which requires the use of a kitchen scale. However, if you search for that same food on a caloric database such as Calorie Count Plus, you may find an alternative form of measurement is provided, such as cups or "portion of package."
Lastly, many foods are available in "single serving" size packages. This means that you can eat all of the food in the package for the listed number of calories. It's important that you don't assume a package is a single serving. Look on the package's nutrition label and find the "servings per package" or "servings per container" section. If it says "1" then you do not need to measure or count the food to know your caloric intake.