And welcome to the Polish forums.
If you are flying in to Krakow (Balice Airport).
I recommend you take the free bus that will bring you to the express light train.
The train will take to the train station in the centre of Krakow (cost 6 Zloty).
You can actually walk to the train, when you get out just follow the road to the right, it is about 500metres.
Buses too are cheap but then you don't know where you are getting off.
There are taxis outside, they are the most expensive mode of transport into the centre but if you have the cash, why not.
I mean do you guys know any good hotels in Kraków? My budget is 350zł -400zł for 5 nights.
I am not sure but for that price, I think you are looking for a hostel, Hotels start from about 200 per night in the centre.
Check this out: polishforums.com/travel-tourism-5/recommendations-krakow-myslenice-27618