does anybody have an idea how/to who you should pay tax to?

does the registration services pay tax for your sales?
do you have to pay income tax?

Easiest answer is: Ask an accountant in your area.

The payment processing services are liable for taxes only on the fees they charge. The money they pay to you is your responsibility.

You will need to prepay taxes based on your projected income and/or income from last year. Of course, if you didn't make any income from selling shareware last year projecting the income you will make isn't obvious.

When and how often you need to send the IRS tax prepayments depends on if you own a business or are simply self-employed (actually, I'm not sure you can simply be self-employed doing shareware but for now let's assume that is an option)

As suggested, find a good CPA. A really nice one might even answer questions (such as this) for free but its not likely.

Aren't you in Israel? If so, I doubt anyone here can answer your question.