How to improve my spoken and written English?

Improving your written English is easy. Post and comment here as often as possible. You will soon grow more confident of the word patterns and coloquial phrases used. You can then take these into your daily life at the supermarket or any other place you go, as has already been suggested.
It is common is some cultures to be ashamed of making a mistake and losing face or status. This is not the case with most English speaking nations. So if are afraid to speak because you may make a mistake, be confident that others will not judge you on your error. Many will understand that you are learning and may even offer advice.

Live for a few months in a country speaking only English.

When I traveled to Japan, I arrived without any knowledge of the language. Within 3 months I could shop in the supermarket , order food in a restaurant, and navigate their public transport system.

It would be cheaper and more effective than any course

Your best shot would probably be to take a short course. There are plenty out there, but it depends where you live.

In the UK, there are lots of courses in and around the London/Greater London area.

Otherwise, there are also lots of online courses you can subscribe to that provide reading material and speaking practice.

Read, Read , Read.
Listen to News Programs, Talk Shows, and yes, horrible as it sounds, Reality TV so that you can keep up to date with the language.
Best wishes.

Immersion is best. If you can't live in a place where English is the common language, having English-speaking friends is the next best thing. Hiring a tutor might be a good idea as well.
Another fun way to learn is to watch DVDs with the subtitles on, so you get both the visual and aural aspects of the language. Reading newspapers is good too. Read as much as you can--it will help immensely.
Good luck!

try rosetta stone..you can buy it anyplace.