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It depends largely on whether the helper
1. is self-employed (i.e.has a registered business)
2. works as your employee
3. does it under the legal radar
The latter is the most common, but since illegal practises are not allowed to be discussed on this forum, you'd have to figure out the risks by yourself.
If you engage a self-employed helper, the pay is freely negotiable and the helper must issue a proper invoice specifying GST number and other business information.
To hire an employee, you have to check whether the person can legally work in Germany, register the work contract and pay tax plus (compulsory) social security deductions to the authorities (below EUR450/month no tax). The minimum wage of EUR8,87/hour applies and you have to grant paid leave and sick days.

So this means if the helper is not from a registered business and is some random person then there would be no issue of gst?
Also where would one need to register the work contract?

If you employ a helper for up to €450/month, you need to register this on and pay the dues there, too.
For higher earnings, things are more complicated and I don't know the details. Possibly the Arbeitsamt and Finanzamt could help you.

Self-employed helpers are available on helpling.de and similar portals.

Can you suggest some more websites for domestic help/cook?

It is very uncommon in Germany to have a personal cook - and will certainly cost more than eating out.
Domestic help here usually means cleaning and laundry.
You can check the usual classifieds websites like quoka.de and kleinanzeigen.ebay.de for informal helpers, or agencies like helpling.de, daheimhelfer.de or betreut.de for the legal sort. (I have no experience with any of the agencies, so cannot recommend.)

Thanks!

Can you suggest some websites for kitchen top installation?

What do you mean by "kitchen top installation"?
Kitchen furniture and appliances can usually be installed (for an extra fee) by the company where you buy it. Otherwise you can engage a carpenter for furniture assembly and an electrician for appliance installation.