1)As the other poster said, Green Taxi, or you can prebook with TaxiDatum online (abit more expensive around 20-22 USD).
2)I visited LIM all the way to Macchu Picchu with minimal Spanish; I would recommend knowing numbers in Spanish so you can barter for taxi rides. Do not pay more than 10 Soles for a local ride (ie, between Mira Flores to Barranco).
3)There is a tour company in MiraFlores (central plaza) Kennedy Park, that has a half day tour also. They go through all of the buroughs around Mira Flores.
4)MiraFlores and Barranco are usually safe. Get a small map of the area from Sheraton. Explore the amazing food.