You're doing pretty well already! Spelling is good, although I might quibble with "as beginner writer" (say "a beginning writer"). Based on what I see people getting into trouble with here, my main tip would be to avoid sentence structures that are too complicated. Take a break if you find yourself getting tied up in nested relative clauses, extended participial or adverbial phrases where it's not clear what they modify, or conjunctions of long phrases where you've forgotten how the first half started by the time you get to the second. So beware of sentences like my last one!

As James said, you're off to a very good start. You know capitalization and punctuation and the pronoun "I". Many beginners don't know even that much! You should know the eight parts of speech, what parts of speech they modify, and what questions they answer. This will help you determine when you need a noun, adjective, or adverb. You should know the capitalization rules: Proper nouns, countries, languages, people's names, day and month names, etc. You should know how to use simple relative clauses (which, that, who, whose, where). You should know how to use conjunctions (FANBOYS and commons subordinating conjunctions). You should keep parallelism in mind (A, B, and C should all be the same parts of speech/same tense, etc.). You should know how to form the six tenses and when to use them. You should know about verb person, number, mood, tenses, and voice, participles, and infinitives. You should know that the present tense has three different forms, each of which has specific uses. They are not interchangeable. Always use a program that spell-checks. That should get you off to a good start.