The online stores are cheapest. Some of them offer a possibility of making an order online and receiving the ordered product in a real shop.
If you already know what exact laptop you want to buy, use one of the price comparing services:
(the best is to use both of them, there are cases when there is a shop in one of them and it isn't present in the other)
They show prices of the product you want from a lot of different online stores.
The most trustworthy online stores with computer equipment are:
I especially recommend X-Kom, they have a really good approach to the customers, and they have also traditional shops in many big cities. But the other ones are also OK and if the laptop you want is cheaper there, you can order it there.
You can also visit shops like Media Markt, Saturn, RTV Euro AGD, it sometimes happens, that you can buy a laptop cheaply there, but the online branches are usually less expensive. They are better for just seeing and watching the laptops. Be careful in terms of what the sellers in such shops recommend, they are often paid by the manufacturers to advertise their products, so that they would for example persuade you to buy an Asus laptop, and not an Acer one (or vice versa), because Asus paid them more. It's better just to look for opinions and reviews on the Internet.
If you are ordering the product via a post or courier service, compare also the price of delivery. Some of the online stores charge also some money for the delivery from a warehouse to their shop, be careful here too. The safest option of the delivery is "za pobraniem"/"przy odbiorze", you pay then to the postman/courier when you are receiving the package with the product. Another options are "przelew" - you are supposed to pay via a bank transfer then (if you don't have a Polish bank account and you don't want to make an international bank transfer, you can also send a money to any bank account at any post office, it costs 3 PLN for a fast option and, if i remember well, 2,90 PLN for a slower option) or by a bank (credit or debit) card. A delivery option which is often possible is also "paczkomaty" - then you receive the parcel in a special machine, they are often located at gas stations and in similar places, but for them the payment "za pobraniem" is usually not possible.
Sometimes you can choose between the post and a courier service. The post is better in that when you are not at home when the postman is delivering the package, you can receive it the next day (or even at the evening if it's still open) at the post office. But it's usually slower than the courier service. In case of the courier service, if you are not at home at the time of the delivery, the courier will try to deliver the package the next working day, but, probably, at the same time, when you may also be unable to receive it. If you cannot be at home when the courier is delivering the parcel, you will probably have to make a trip to their logistic centre, which is usually one for a very big area and it's often located at the city outskirts.
In terms of the keyboard, there will be no problems. In Poland, English (or rather American, it's slightly different from the British one, but the differences are really small) keyboard is used.