There is nothing like being bilingual. If you speak only one language, you may not know what you are missing. Being bilingual opens up different pathways of thinking, of expression and of being. Bilingualism allows communication in unusual circumstances and with people who would normally be out of reach due to a wall of misunderstanding. Plus, speaking more than one language is a great thing to put on a resume. You can also impress others in social contexts.
The first step is to commit to learning another language. It will take time and dedication. It may involve expenditures. It will have its ups and downs. Without a commitment, you cannot guarantee you will become bilingual.
You've made a commitment to become bilingual. Now, decide which language you want to learn. For some people, this will be an easy choice. Perhaps members of your family speak a foreign language. Perhaps you have a boyfriend or girlfriend from another country. Maybe you desire to travel and live in a faraway place. Maybe you want to learn a language for business purposes. These factors or others will determine which language you want to learn. Keep in mind some languages are much easier to learn than others. For native speakers of English, Spanish is easier to learn than, say, Chinese.
You're committed to becoming bilingual and you've decided which language you want to learn. Now, choose your path. Do you have access to native speakers? Can you find nearby classes? Are there learning materials at the library or bookstore? Can you find online content for the language? Are there newspapers or other publications available? Online radio stations or even local radio stations? You need to find which resources are available so you can design the path by which you will learn the language. The more resources available, the better.
Start right away. It will take a long time to become fully bilingual and the sooner you start, the sooner you will get there. The younger you are, the easier it will be. Don't put it off.
Now that you have a path, invest time and resources. If you can take a class, great. If you can listen to the radio in your target language, great. Buy some books. Read an online newspaper. Ask questions of a friend or neighbor who speaks the language. The important thing is to invest your time and effort (and money) into learning the language.
Don't let a day go by without learning a new word, practicing your listening comprehension, watching a movie in the target language, conversing with someone or anything else that will improve your capability. You don't have to spend four hours a day, but you do have to spend 15 minutes a day as a minimum.
Visit a part of the world where your language is spoken. Your capabilities will improve much, much faster in an environment of immersion than in your own home country.
After a year, analyze where you are. Make a chart of your progress. Can you speak more than before? Can you read more than before? Can you understand more than before? How is your vocabulary progressing? See how far you have come. Do this every six or 12 months. It may take five years to reach your destination, but you will get there slowly but surely, as long as you keep at it.