It's easiest if you understand the coding systems that are used to categorize commodities for which trade data is collected. At any one time there are several systems, and the coding used to report certain statistics from a certain government will change over time as well. Please see the tab on coding systems.
Current data are available electronically, although if you just need a simple aggregate number a paper resource may be as easy to use. Historical data may be available only in paper or microfiche. Be aware on whether you're looking for total country-to-country trade data, data on total imports or exports of a particular commodity into or out of a specific country, or detailed country-to-country data for a specific commodity. Please see the tab on sources for trade data.
If you need to choose a commodity to study for a class assignment, scanning through a list of commodity codes is a good idea to find a cleanly defined product. You might be frustrated if you try to settle on a product off the top of your head, since it might not be cleanly defined, so it's more pragmatic (for the purposes of a class assignment) to choose a product based on the data available for it.
If you need to study trends over time, keep in mind whether a product has existed very long. Remember that it takes some time for a new product to be given its own numerical code and show up in trade data, so for a while it will be lumped in with similar products. Modern electronic products are difficult to focus on or to follow back many years, especially if the product is built of subcomponents, each of which may be coded separately.
Products that are commonly taxed for political reasons will be broken down into detailed categories in tariff schedules. This includes textile products, alcohol products, and jewelry. This is especially a problem when data is only reported at, say, the 10-digit HS level of detail and not aggregated to fewer number of digits (although most data extraction products do allow different levels of aggregation). There might be dozens of categories of wine, based on alcohol content, whether in bottles or casks, whether white or red, whether normal or reinforced, etc.