Well, the first thing is to read more. It helps a lot. And if you are given lots of time when you write your essay (especially when it's homework), then try to brainstorm or make out a mind map. Let's say you have a topic on a country. You just need your imagination, a pen and a piece of scrap paper. Write down what you can think of that is on the topic, like the country's food, culture, places, specialities, population etc. You can search information on the internet, and that comes in handy when you need lots of information. You can also ask for first hand information from people. Maybe you have a relative there or maybe you can go online and ask one of your online buddies for their opinion on their country.
After you did all the information collecting, interviewing and brainstorming, you would have already increased your own knowledge on the topic. So just sit down, and try to write your draft. Introduce the country first, where it is in the world, and maybe how long it's history. Then maybe give the reader some background information before settling on what you're supposed to talk about. It's city life, the shops, the people, the food etc. Remember to write drafts before you really write the thing out and hand it in. Drafts are extremely useful and if you can put them on the internet (such as here) and people can read it and give you their opinions, that would be even better. Read what people think and try to make changes. Then your essay would be really good.
P.S, remember to proof read your essay.