The following maps should help you get your bearings.

Signposting on the Island is reasonable, but many of the minor roads are single track and look alike, so it helps to have a navigator.
Most major roads lead to Newport, from where you should take the Shorwell road (turn left in Carisbrooke) to head for Brighstone.
If you are coming from Lymington via Yarmouth, you may like to take the A3055 coast road via Freshwater,
as it has fine views over the Channel. This is known locally as the Military Road, so called because it was built by German prisoners during the first world war. Turn left at Brook to join the B3399 to Brighstone.

More (zoomable) detail here