Many use a price from a prominent bitcoin exchange, or the CoinDesk Bitcoin Price Index.
Some sellers apply a percentage on top of these rates to cover costs and as a convenience/anonymity premium.
You could use a mobile app to calculate prices. Popular apps include Zeroblock and BTCreport.
It helps to be aware of local fluctuations in price. Price can vary from country to country, often due to difficulties in obtaining bitcoin with the local national currency.
There are many bitcoin meetups around the world where people are happy to trade bitcoin and other cryptocurrencies.