Я учусь шить.

Because учиться means to learn / to study, while учить usually means to learn something by studying it or to teach.

So:

• Я учусь шить = I am learning to sew
• Я учу русский = I am studying/learning Russian
• Я учу детей = I teach children

In standard Russian, if you mean I am learning to do something, you normally use учиться + infinitive:

• учусь готовить = I’m learning to cook
• учусь водить = I’m learning to drive

The -сь is the reflexive ending, from -ся / -сь.

The dictionary form is учиться. When you conjugate it:

• я учусь
• ты учишься
• он/она учится

This reflexive form is part of the verb itself. You should learn учиться as a whole verb, not as учить plus some extra word.

In this sentence, учусь means I am learning / I study.

Because Russian commonly uses учиться + infinitive to express learning to do something.

So:

• Я учусь шить = I am learning to sew
• Она учится плавать = She is learning to swim
• Мы учимся говорить по-русски = We are learning to speak Russian

English uses to sew, and Russian uses the infinitive шить.

No. Russian does not need an extra word here.

• учусь шить already means am learning to sew
• You do not need a separate word for to

And unlike English, Russian also usually does not need a word for how to in this pattern.
So Я учусь шить is the natural way to say it.

If you added как, that would change the structure and usually sound more specific:

• Я учусь, как шить... = I’m learning how to sew...

But in a simple sentence, Я учусь шить is the normal choice.

Russian does not use a separate present-tense verb to be in sentences like this.

Also, Russian has no special continuous form like English I am learning. The simple present often covers both:

• I learn
• I am learning

So Я учусь can mean:

• I learn
• I am learning
• I study

The exact meaning depends on context. In this sentence, it is naturally understood as I am learning.

Yes, very often.

Because the ending -усь already shows that the subject is I, Russian often drops the pronoun when it is clear:

• Я учусь шить
• Учусь шить

Both are correct.
Including Я can make the subject clearer or more emphatic, but it is not always necessary.

Not completely. Russian word order is more flexible than English.

The neutral order is:

• Я учусь шить.

But other orders are possible for emphasis:

• Учусь шить. = neutral, with I omitted
• Шить я учусь. = emphasis on sew
• Я шить учусь. = also possible, with emphasis shaped by context

For a learner, Я учусь шить is the safest and most natural basic order.

Шить is pronounced roughly like shyt’, not like English sheet.

A useful point: after ш, Russian writes и, but the sound is closer to ы than to a clear English ee sound. This is part of a spelling rule.

So:

• written: шить
• pronounced approximately: shyt’

Also, the final ть is soft, so the word ends with a soft t sound.

Yes, in a general sense it can be understood that way.

The sentence focuses on learning the skill:

• Я учусь шить = I’m learning to sew

In natural English, I’m learning to sew is usually the best translation.
I study sewing is possible in some contexts, but it sounds more like a school subject in English.

So the Russian sentence is about acquiring the ability, not just studying theory.

Not if you mean I am learning to sew.

In standard Russian, Я учу шить would normally mean something like:

• I teach someone to sew

The object may be omitted if it is understood from context.

If you mean that you are the learner, use:

• Я учусь шить

That is the standard and natural choice.