Mobile Phone in the Netherlands
The major mobile phone companies in the Netherlands are T-mobile, KPN and Vodafone. Of course, you could keep using your own phone (with your subscription) but this is probably pricy and unnecessary (if you still want people to be able to reach you on your previous phone number you can always get a phone that allows dual-SIMcard) usage. At the end of this article, you can also find two price comparison tool to get the best deals (SIM-only and contracts including a phone).
Phone contract or pre-paid phone
For both options, you need to show your passport (or another form of ID) when you buy your new SIM-card. And when you are getting a subscription you often have to show a receipt of your bank account (or contract).
The costs are determined by your usage, if you are calling international or not, how much internet you use, duration of the contract etc.
SIM-only or contract
In addition to contract and pre-paid, you can also choose if you want to have a SIM-only or a phone included (which will be reflected in the price of course). Please check with your new employer if you get a phone with your job, this is often the case when you need to use your phone for work as well.
It is also worth noting, that in order to report a stolen phone, you would require to provide the officials with the unique IMEI/MEID number of your phone.