Learn a few important phrases at first, and learn how to pronounce them well. When you're confident with these, you can choose a few more to practise. Start with simple phrases first, then go on to more complicated ones.
For example, learn key phrases for saying hello and greeting someone:
"Hi! How are you?"
"Fine thanks. And you?"
Or a simple phrase to ask directions:
"Excuse me! I'm looking for ... (London Road)"
Or a simple phrase to order food in a restaurant:
"I'd like the ... (fish and chips) please."
You don't need to speak a lot to express yourself. In many situations, such as travelling around, shopping, phoning, etc, you don't have the time to have a long conversation. Concentrate on communicating the essential part of your message until you start feeling more confident.
This is probably the most important tip. The more you practise, the better you get - and the more confident you will become. When you practise important phrases, they become natural and easy for you. If you aren't in an English-speaking country, you can go to conversation clubs, or go online via a Google+ hangout, for example. (The Speak English! ebook has a list of places where you can chat for free online.)
You don't need complicated grammar or sentence structure. If your sentences are short and grammatically simple, you are less likely to make mistakes or confuse the person you're talking to.
Sentence structure tip!
Remember the SVOMPT word order rule:
Subject
Verb
Object
Manner (How)
Place
Time
For example:
I (subject) like (verb) walking (object) fast (manner) in the forest (place) in winter (time).
Remember: not every sentence has all six components, and you can omit manner, place and time and still have correct sentence structure.
When you need to communicate a message, you will find the way to do it! For example, you can use gestures to help you explain to another person. If you are in a train and feel hot, you could say "Can I open the..." and gesture towards the window if you can't remember the word for "window".
A great way to improve your confidence is to watch other people speak English. What phrases do they use? What gestures do they use? How do they respond to each other? Perhaps they use special phrases or particular intonation. Observing how other people communicate is a great way to extend your own "library" of useful phrases for different situations. If you don't live in an English-speaking country, check out film and TV, or trailers on YouTube, for example.
Use your phone to record yourself speaking phrases or words. When you listen back to yourself, how do you sound? Don't be too critical about your accent - it doesn't matter if you sound like a non-native speaker. But it's important to speak clearly. Recording yourself helps you become more aware of how you speak English, and you'll begin to understand which English sounds are most difficult for you. Then, with a little practice, you can soon get better at saying these difficult sounds.
If you're nervous or embarrassed, you might speak too fast - or too quietly. It's normal to feel worried, especially in new situations. But concentrate on those few phrases and try to speak at normal speed and volume. When you see the other person responding naturally to you, you'll know that what you say is good English!
Some days it's easier to speak English than other days. Sometimes you can find the words and phrases you need without any problems, but other times everything is really difficult. Some days you never seem to make mistakes - and then the next day it's all wrong! But don't worry. This is completely normal! The important thing is to understand that tomorrow is a different day. Keep practising and you will improve your speaking. Don't worry too much about making mistakes, either. They help you improve your English.
Native speakers will rarely correct your grammar, but they'll probably correct your vocabulary and pronunciation. You can even ask, "Sorry, what do you call this in English?" and people are usually very happy to help you.