You can reach Isla del Sol by ferry from either Copacabana or Yampupata, or with a guided tour.
Ferry tickets may be purchased at the Asociación Unión Marines ticket kiosks on the beach in Copacabana or from town agencies (save yourself the trouble and buy direct). Boats leave at 8:30am and 1:30pm from the beach and land at either Pilko Kaina or Escalera del Inca near Yumani (one-way/round-trip B$20/30). Return trips leave Yumani at 10:30am and between 3pm and 4pm (one-way B$25).
Titicaca Tours will take you to both Isla del Sol and Isla de la Luna in half a day – two hours to explore the former and one hour at the latter – for B$30. However, it's highly recommended to stay overnight or longer on one of the islands.
Asociación Transport Yampu Tour Lacustre runs boats between Yampupata and Isla del Sol for B$130. Prices are per boat.