I'll introducem going to talk about my favourite subjetct at school.

More natural English like this.

It was math when I don't learn graphs, though not if we were learning Geometry.

Most of the English speaking world say "maths" though Americans say "math"! I assume you mean Geometry by "graph".

But my mainMy new favourite subject is sSocial suStudies.

Sentences don't start with 'but' in English and "main favourite" doesn't sound natural. Also is "studies" not "sutudies".

Bbecause I like geography very much.

If you mean you like SS because of the geography element the last 2 sentences would be combined into one. In English you never start a sentence with 'Because'.

I'll introduce my favorite subject.

It wasused to be math when I doidn't learn graphs.

(1) "Was" is okay, but I think "used to be" is better.
(2) It should be "didn't" and not "don't", since you're talking about a time in the past (when you learned graphs). I think that's what you meant at least.
(3) It should be "graphs" and not "graph", since you're referring to graphs in general, rather than a specific one (presumably).

But my main favorite subject is mainly social sutudies.

"Main favourite subject" doesn't sound very natural in my opinion.

I'll introduce my favorite subject.

It was math when, although I don't learnike learning to graph.

I hope I understood what you're saying here.

But my main favorite subject is social sutudies because I like geography very much.

If feels more natural to combine this sentence with the next one, since the two ideas are directly related.

Because I like geography very much.