Recent timetable changes have reduced the services along the North coast. Using the travelplanner below will show bus A5 from Airport to Padstow, bus 11 to Wadebridge then bus 95 to Tintagel. A later option is to travel into Newquay Town centre to intercept the 95 at St Columb.
The bus changes can be made at various points, so I think it best to obtain the bus timetables and make your own decision where to change. For example you may prefer to travel into Padstow centre and stay over lunch time rather than change at Tesco on the outskirts.
Are you walking to Newquay over the week-end? The SWCPA suggests 2 days for the walk, about 44 miles and the first day is pretty tough. Is accommodation arranged?
Mobile signal is patchy so if you are walking alone make sure someone knows where you are and what time you will arrive at your accommodation. Sorry if that is obvious.