How would one invest in things for example (wheat, oil, gold, silver, platinum) and other commodities? Does E-trade offer a method of doing this?
I agree with QuantK the easiest way is through an investment in ETFs.
The other possibilities are:
Buy commodities in the spot market.
i.e. you just buy the commodity you want and hold on to it. This implies that you will need to store it, which you probably don't. Hence, the next alternative.
Investing in futures is not something you should consider without doing your homework first! Futures positions are taken on margin and are thus "bought" with leverage.
Furthermore, futures contract have a maturity date. This means that you would need to "roll" your position if you want to keep exposure to the commodity. In addition you should note that spot market returns are not necessarily the same as movements in futures returns due to basis risk and differences in timing (futures typically converge to the spot price the closer to maturity but in essence reflect price discovery over time).
If you want to invest in commodities through taking a position in futures contracts I advise you to become familiar with at least the following concepts: roll returns (backwardation/contango term structures), margin (especially what kind of collateral your broker accepts) and transaction costs.
ETFs typically invest in commodities through one of the above stated possibilities. They either arrange storage or they make sure futures positions are rolled. If ETFs invest through futures your return typically consist of the futures return and the return on collateral (typically some bond portfolio).
There are also other other derivates which you can consider but in the end these are typically linked to futures returns.