Don't rush. Learning a new language takes time. Speak each word slowly, and sound out your consonants and long vowel sounds. Speaking slowly and methodically will make your spoken English easier to understand. You make more mistakes when you're nervous and trying to speak quickly. Strive for accuracy, not speed.
Invest in good dictionaries. At first, you'll want a bilingual dictionary. As you progress, you should use an English-English dictionary that gives clear definitions and provides context for how English words may be used.
Read as much English as you can, whether it is in books, newspapers, magazines or online. Watch television programs, movies or online media in which English is spoken. When you search for something on the Internet, search for it in English. The more you read, listen and study in English, the more deeply you will absorb it.
Learn new words, phrases and grammatical constructions every day. Practice them by yourself at first, then with other people. Record yourself speaking, and play back the recording and analyze it for mistakes. The more you practice, the more natural the new words, phrases and constructions will feel, and the more confident you will be in saying them.
Get plenty of practice by interacting with native speakers, whether in person or online. The more exposure you have to English as spoken by native speakers, the more it will integrate itself into your mind.
Strive to think in English instead of in your native language. If you get into the habit of thinking in English, then speaking English will come more naturally as well.