It's true that you can't date sedimentary rocks directly using radiometric dating because the rocks that make them up were preexisting. You can use radiometric dating to date ash deposits or basalt (lava) layers that frequently can be found covering already deposited sediments. So, since we know that newer rocks are deposited on top of older rocks and any geologic structure that cuts through another structure has to be newer than what it cut through (i.e. magma coming up through existing sedimentary rocks has to be newer than the sedimentary rocks) we can use those igneous rocks to tell us how old the sedimentary rocks are, or at least the age range they fall into. That dates the fossils in those rocks as well. There are several isotopes that are used for radiometric dating, some are more accurate for shorter durations, like carbon-14, and others like Uranium have isotopes that can be used to approximate ages into the billions.
The oldest rocks on earth that we can date are 3.8 billion years old. Before that, the Earth is assumed to be molten. We get the 4.5 billion year age of the earth by dating rocks that were around at the time that the earth, and other planets in the solar system, were forming. These are easy to find, the fall to earth all the time. The age of most of the meteorites that fall to earth is about 4.5 billion years. I know you didn't ask, but I thought I would throw that in there.
Thank god for the disable signatures option.
