To be honest,If I keep to improving my English, T1) If my English keeps getting better, it's thanks to you. ¶
2) If my English keeps getting better, it's thanks to you, my friend.

I'm assuming this person would already know that they're your friend, so I omitted it for the first suggestion. However, if you want to keep that idea in your sentence, see number 2.

Or I sayI could also say the following: I owe you for improving my English.

You could combine the next sentence and this one together.

I owe you because I keep to improving my English.

The structure "to improving" (which is an infinitive structure combined with a gerund) is not used in English. At least, if it is, I haven't encountered any native speakers who use it often. Infinitive structures are (to + verb), which convert verb forms into nouns, and gerunds are (verb base form + "ing"), which indicate that you are currently doing said action that relates to that verb. Here's a guide on infinitives. https://www.perfect-english-grammar.com/infinitives-of-purpose.html

I don't know which one theis best to use it?

Both of your sentences emphasize gratitude, but the one that begins with "I owe you" has the added layer of feeling indebted to your friend. It gives off the feeling that you want to do them a favor in turn for all the help they've given you.

To be honest,If I keep to improving my English, T if my English is improving, it's all thanks to you my friend.

Or should I say.:

I owe you because I keep tofor helping me improvinge my English.

I don't know which one would be the best to use it?