The answers to this question will depend somewhat on the tone that you are trying to achieve with your blog. I will attempt to answer each question below.
That very much depends on the specifics of the case. I wouldn't repeat a word unnecessarily, but sometimes it's more or less unavoidable (like in the first sentence of your question). I wouldn't worry about this too much.
Since you are writing a blog, the tense can be much more varied than it would be in a technical paper or something of that ilk. Basically, just use whatever tense you would use if you were speaking. Make sure to follow grammatical rules, though (e.g. make sure tenses agree within sentences).
It does not matter whether you use American or British spellings/word-choices; however, you should choose one and stick with it consistently. It just seems weird to readers if that gets switched around within a post.
This depends on the level of formality that you are trying to achieve with your writing. For a blog, I don't think anybody would think twice about a contraction here and there.
Google Translate is good for translating individual words, but not great for translating whole sentences, and definitely not great for translating an entire article. It could work to type English words into Google Translate and see if they translate back to your native language the way you want them too, but even that won't be foolproof. The best thing to do would be to ask a native English speaker.
You made a few mistakes in your question, but none of them are mistakes that I would consider unusual for a non-native speaker.
- I plan write an IT blog in english language -> I plan to write an IT blog in English.
(The word "language" at the end of the sentence is not needed. If you want to include it, you could say "the English language.")
(We do not use the word "advice" as a singular noun. Instead, we would say "Any advice" or "All advice.")
Is it good to use two (or more) same words in one sentence? -> Is it good to use two (or more) of the same words in one sentence?
Should I use only one tense in article -> Should I use only one tense in an article
Should I use british or american dialect of english -> Should I use a British or an American dialect of English
("British", "American", and "English" are proper nouns, so they are capitalized.)
(Not really a mistake here, but these "shortcuts" are called contractions in English.)
- Do I commited a lot of mistakes in the above content? -> Did I commit a lot of mistakes in the above content?