Does JotForm charge fees when processing a payment through Paypal?

Do you know how much paypal takes from the payment and if jotform too takes a little %?

Seeing that you're based in UK, refer to this help page from PayPal regarding the fees:

https://www.paypal.com/uk/webapps/mpp/paypal-fees

From our end, we don't charge you at all nor do we process any payments. All these will be done through the Payment Processor you're using (which is PayPal in your case). The only thing you will be charged for would be your JotForm subscription if you opted to upgrade to a paid subscription.

Many thanks great and fast support, congratulation!

Just last thing: in the form when it says "each month" or each week" it means it takes payment automatically from my customers each months?

That's correct, each month would mean a monthly subscription (charged each month) and each week would mean a weekly subscription (charged each week).

These are the subscriptions you have setup on your Paypal payment field though so whatever subscription you add there will be the ones reflected on your form.

To have a better understanding, I'm linking a guide below that will serve as a great starting point.

Related guides: 

PayPal-Forms 

How-to-Setup-a-Recurring-Payment-Option