Understanding Sidney's sonnets requires more than a simple reading. In some, the syntactical alterations need to be mastered; in some the structure needs to be understood; in some their place in a sequence of other sonnets needs to grasped. In this Sonnet 15, vocabulary and allusions need to be attended to. Your understanding of this sonnet will be forestalled until you understand:
"Purling" is the motion and noise a flowing spring or river makes: "search for every purling spring." Parnassus is a mountain that rises above Delphi, the center of Greek oracles from the gods; it was beloved by Apollo and the location of the fountain associated with the Muses called Castalia. Note these connections in "purling springs / Which from the ribs of old Parnassus flows."
"Denizen" has an obscure definition meaning something "adapted to a new place, condition" (Random House Dictionary). Astrophil, the poetic speaker, is addressing inferior English poets who borrow from Italian sonneteer Petrarch and newly adapt his "long-deceased woes" to their own native wit. "Far-fet" is an archaic abbreviation of "farfetched": improbable, forced, strained (Random House). "Endite" is either "to write" or "to dictate."
So now the meaning of the sonnet emerges. Astrophil is complaining against inferior poets who imitate Petrarch and do it badly and in so doing steal his thoughts and words: "And sure at length stol’n goods do come to light." He complains as he does about inferior sonnets "running in rattling rows" because they "do bewray [disclose] a want of inward touch." But what has this to do with Stella?
The volta at line 9 turns from his complaint to his assertion that "You do wrong ...." The ending couplet--which is a form Petrarch is said never to have used to end a sonnet--resolves the dilemma of the inferior poets by advising them that if they but turn their attention to Stella, they will be inspired with truth and pure wit and then they may begin to write (or dictate to a scribe ...):
But if (both for your love and skill) your name
You seek to nurse at fullest breasts of Fame,
Stella behold, and then begin to endite.
(Lines 12-14; couplet 13-14)