Every Future contract has a different size and necessary margins (both initial and maintenance margins).
As one can see on the following lists, some futures commodities are very accessible to small portfolios while others are more risky and dedicated to big portfolios...
Obviously, rookies should start with trading e-minis first and "cheap" commodities such as Mini-Corn, Mini-Wheat, Sugar or Mini-Gold and Silver. Gas should be left to more experienced traders (those commodities are hot and very volatile !)

Trading Commodities is not so common among individual investors or daytraders. Most people prefer trading stocks, indexes or even currencies on the Forex. Commodities are probably the most interesting products to trade though, simply because they are highly related to Geopolitical issues and to the most simple indicator : supply and demand.

Natural gas is a fossil fuel that is colorless, shapeless, and odorless in its pure form. It is a mixture of hydrocarbon gases formed primarily of methane, but it can also include ethane, propane, butane, and pentane. Natural gas is combustible, clean burning, and gives off a great deal of energy.

Sugarcane is cultivated in tropical and subtropical regions around the world roughly between the Tropics of Cancer and Capricorn. It grows best in hot, wet climates where there is heavy rainfall followed by a dry season. On a commercial basis sugarcane is not grown from seeds but from cuttings or pieces of the stalk.

Sugar beets, which are produced in temperate or colder climates, are annuals grown from seeds.

Frozen Concentrated Orange Juice (FCOJ) was developed in 1945, which led to oranges becoming the main fruit crop in the US. The world’s largest producer of orange juice is Brazil (Sao Paulo), followed by Florida.

GRAIN (futuresource.com)
GRAIN & OILSEED(tfc-charts.com)
Corn / Wheat / Oats / Soybeans / Soybean Oil...

FOOD/ FIBER/SOFT (futuresource.com)
FOOD & FIBER (tfc-charts.com)
Coffee / Cocoa / Sugar #11 / Sugar #14 / Cotton Orange Juice...
METALS (futuresource.com)
METALS (tfc-charts.com)Gold / Silver / Copper / Platinum / Aluminum / Palladium
ENERGY(futuresource.com)
ENERGY (tfc-charts.com)Light Crude Oil / Heating Oil / Propane Gas / Natural Gas / Brent Crude Oil / Electricity

Useful links and readings about Commodities fundamentals