If you are not of pensionable age or have an exportable disablitly payment then you will need private health insurance.
Several companies provide this cover which must give you a minimum of the cover provided by the Spanish health service. Just Google health insurance Spain and you can compare quotes.
Note that the health insurance will, generally, not cover any pre existing conditions and you will have to pay for prescriptions.
Once you have been resident for 12 months or more, you can opt to use the special agreement by paying 65 euro a month (each and 145 euro a month if over 65) into the health system if that is still going. Again, it doesn't cover the cost of prescriptions.