Dont want to sound condescending, but if you didn't know the A-series is a over head valve rather than over head cam, it may pay to, either pay a machanic to change the head gasket or observe someone who has done the job before.

First thing to do before anything else is to buy yourself the workshop manual.

Its not particularily a difficult job, but can be easily stuffed up, if you are unsure of what you are doing. The only specialty tool you will need is a torque wrench to torque up the head bolts. Speaking of head bolts, they can be re-used, providing the thread section is in good order.