It says that Schedule C allows to deduct
Partial cost of your cell phone.
Partial cost of your personally-owned computer/laptop (if used for business).
Partial cost of your Internet Service provider (if used for business).
Business use of your home/apartment

How is "Partial" calculated in case of running a company from home?

If it is a one person company (the company was officially registered) - is it a self-employment?

Thank you very much

It pro-rated based on personnel versus business use.

It IS considered self-employment.

Thank you

You wrote "It pro-rated based on personnel versus business use."

Should I decide on my own how to prorate it, or there are special rules?
Thank you again

There are no special rules.

Just be comfortable with the percentage of business use you pick, i.e. be ready to defend it in an audit.

What kind of a company do you have?

One person software start-up. Does it make a difference for tax purposes?
Thank you

It CAN make a difference. Issues too complex to discuss here.

Go Ye Quickly and See a Tax Pro! :-)