There is one thread here that suggests learning grammar is less important than anything else. Contrary to this ill-advice, like in other languages grammar is very crucial in English language.

Firstly, grammar is all about a tool with which you structure your information. For instance, if you want to tell people what happened yesterday and expect them to understand what do you want to say, then you can't use future tense form.

Secondly, your mastery of grammar usage and spelling will immensely help you to speak and write better in English. Like any other languages, there should be clear who is the subject and what does the subject do and when does it happen. For example, the sentence like this, I GO is not a complete sentence as it doesn't explain much about the subject. Where does he go?

Thirdly, an old adage says that practice makes perfect. That's true. But what is a good way to practice? I reckon you should start with a few simple steps.

1. Compose a few simple sentences and write them down. Bit by bit you develop them into a paragraph. Do not try to explain things that are far too complicated, particularly if your vocabulary is very limited.

2. Read simple books. Children's books are great. They consist of simple sentences and over time you move to more complicated story.

3. Form a discussion group. Ask your friends who are interested in learning English to form a discussion group. You can organise a weekly or fortnightly meeting, or whenever suits you best. Speaking in a group will not only help to boost your confidence but also exchange ideas with likeminded people.

4. Don't try to mix up language ie English and Indonesian. They both have a totally different structure. If you mix it up, you might end up talking in English with Indonesian grammar.

Good Luck!