This is a simple outline plan for a wooden pergola measuring 450cm x 360cm.
Tanalised (pressure treated with preservative) wood should give a life expectancy in excess of 20 years.
Posts 12cm x 12 cm x 300cm. (A)
Cross Rail 6 cm x 12 cm x 360 cm. (B)
Long Rail 7cm x 14.5 cm x 450cm. (C)
Those timber sizes are taken from locally available materials though you should be able to find equivalent lengths of timber.
Posts should be inserted into the ground around 50cm to stabilise the construction and concreted in the case of soft ground.
The long rails can be fixed to the top of the posts with bolts for additional strength.
Fixing the cross rails to the long rails could be accomplished by utilising half lap joints, joint hangers or similar devices.