Hi there, and welcome to the forum.
1- Practice! You can start by practising alone. Try reading texts or conversations out loud to yourself. Then later, perhaps, record yourself, and listen to the playback to improve your pronunciation.
2- There's no need to be nervous about speaking in English. Do not be put off by any mistakes you make. Learning from mistakes is an important part of the process.
3- Try finding someone to practice your speaking with. Maybe you can join some online group or community with a common interest (hobby, sport, etc.) and you can talk with them.
I don't know where in Sri Lanka you live, but I just Googled "English speaking clubs in Sri Lanka" and got plenty of results. Have a look yourself and see if there's a local one.