First, decide how much you want to spend on a phone. Think globally: look at all your expenses; the phone is only one of them. Choose a monthly amount taking your income and other expenses into consideration.

Then assess how long you use the phone every day, week or month. This will help you choose the best plan to suit your needs. If you choose a restrictive plan and keep overspending it, it can be costly. On the other hand, if you are always well under your plan, you probably pay for minutes you never use.

Then compare the different providers' offers and choose the one which suits your budget (how much you want to spend on your phone) and your needs.