These days the solder does not contain lead - health and safety and all that - as lead and flux fumes are pretty nasty things. Get yourself a small tin of paste flux. Before putting solder on the wire or the terminal, a small blob of flux is all you need. Now bring in the iron, wire and solder and so as you usually do. Flux paste will help remove the oxidised layer from the solder, and give you a nice shiny solid joint first time. HTH
PS - do not use Bakers or any other type of liquid acid flux, as that will eat into the parts and is not good!!