It can be hard to express one's opinion when you lack some of the special vocabulary used in the field that you want to speak about. There's no way around learning it. So for example if you want to be able to express your thoughts on politics in English, you should start reading about politics in English, and try to debate about it in English. Same with any other topic. Reading material could be everything from books, news, websites, videos etc. To practice expressing your opinion with newly learned words, you could try writing short opinion pieces and posting them here to get feedback (just keep the community guidelines in mind, some topics aren't allowed), you could use internet forums or try and find a native speaker interested in the same topic to talk to. I actually learned many expressions and idioms when I was still using Reddit daily. It was easy to find the communities discussing topics I'm interested in. Another idea I have is joining a debate club, but I didn't try it myself.