It may beThis might sound rarstrange, but it is difficult for me to selectchoose a theme for writing. This might sound strange, but it is difficult for me to choose a theme for writing.

Rare = uncommon, infrequent, for example a rare animal or a rare book. Strange = unusual, weird, odd, peculiar. Select implies that you are choosing from a pre-determined range of options, whereas choose is more general.

Maybe, fFor this reason, / Maybe this is why I have avoided practicing my English, and I have not advanced in the learning of this language. For this reason, / Maybe this is why I have avoided practicing my English, and have not advanced in learning this language.

You don't need to repeat "I" in the second part of the sentence, as it is obvious that you are speaking about yourself.

Now, I think that this is the time to begin, although some themesubjects that I will selectchoose, like politics or religion, maybe doaren't likeinteresting to some people. I think that this is the time to begin, although some subjects that I will choose, like politics or religion, maybe aren't interesting to some people.

Theme has many synonyms to diversify your language - eg. subject, topic...