"There is one chair."
At one point Japanese adopted a lot of aspects of the Chinese language (including the kanji of course), so you get this situation where a character can have multiple readings - kun (Japanese) and on (Chinese) pronunciations. Which one you use depends on the context
So you've learned the basic numbers, いち、に、さん etc. - these are actually on readings, "Chinese numbers" if you like. They're the readings you usually use, so don't worry about it too much. But Japanese has its own original numbers: ひとつ、ふたつ、みつ and so on. They're not used much now, but they do pop up in a few places - and counting objects is where you'll tend to see them
The main one is the generic counter - one thing is ひとつ, two things is ふたつ. In Japanese when you want to count things, you have a number and then some "counter" word that describes the object - "person", "long thing" and so on. The generic counter is for when you want to give a count without getting into what it is
Another place you'll see them is with the counter for people - 一人 is ひとり, 二人 is ふたり, 三人 is... さんにん! Yes it's back to the Chinese on readings, with a different reading for the counter too (‐り is kun, にん is on) - this probably reflects that Japanese stuck with the old way of referring to one or two people because it's more tightly woven into the language (ひとりで means alone for example), but adopted the Chinese version for counting in general.
It's more complicated than that (languages usually are) and I'm definitely sketchy on the history, I just wanted to give you a general feel for why you sometimes get different readings for things. Gennnnnerally single-characters will use the kun reading, and multiple kanji represent a word from Chinese and use the on reading. So you can expect each character to have at least two pronunciations, but it depends on the context!
I haven't tried it on this one in particular, but what I've seen with these counting exercises is that if you want to put the number before the noun (as you propose), you must link them with then particle の, therefore: 一つ の 椅子があります.
And for completeness's sake, it's pronounced "hitotsu" not "itsu".
I hope this helps!
is it possible to add a そこは in the beginning, to make clear what the topic is? I'm getting realy confused with this concept of a topic marker, and when I need to imply a certain article, and writing it down as a hole, altough Japanese would skip this part, would help me to learn (if its right, because, since Duolingo doesn't except it, I'm not even sure about that)
I would say that the English sentence "There is one chair" doesn't imply anything about location, despite the use of the word "there" -- it really just means "one chair exists" without telling you where it is. (Like French "il y a", not "voilà".) So, in the absence of further context, it would not be appropriate to specify そこ (or ここ or あそこ) as you propose.
Now if the sentence was "here is one chair", or "there is one chair there", that would be different.