Do I have to sign up for a fixed contract with all of them?
No, each offers a non contract package/s or pay as you go schemes. Each provider has different benefits. I.E. certain number of free SMS's, cheap calls abroad, free calls to certain numbers etc. You will just need to see which is best for you.
You will be able to go into most mobile phone shops and buy a pre-paid SIM card for your chosen network.