The meaning of 'upasishye' and 'preetyartam'.
"Upashishye" means "I am worshipping" Or "I'm performing the Upasana". "Sandhyamupashishye" means "I'm worshipping Sandhya" or "I'm performing Sandhya worship".
You can get a Sankalpa Mantra from the book "Essence of Dharma Sindhu":
After Pranayama as above, then Gayatri Avahana Sankalpa states: Mamopaattha Samasta duritakshaya dwara Shri Parameswara preetyartham Praatah Sandhyaamupaasishy/ or Maadhyaahnikam Karishey/ or Saayam Sandhyaamupaasishey (May I begin to worship Goddess Sandhya in the morning/afternoon/evening by destroying all of my sins by the grace of Parameswara).
This is a Smartha version of the Sankalpa. That's why they say "Sri Parameswara Prityartham" (for pleasing Lord Shiva). Vaishnavas, in that place, will say "Sri Narayana Prityartham" (for pleasing Lord Vishnu).
So, "Prityartham" means "for pleasing" or "for appeasing".