Lets start with the U-235 device. You will need about 60 kg of almost pure U-235 (over 90 %). Natural Uranium consists of only about 0,72 % U-235, the rests is U-238, wich not is fissionable.
Here is a drawing of the Uranium bomb "Little Boy" which was dropped on Hiroshima.
A Plutonium bomb is more complicated, it is built on a implosion of a Plutonium-sphere. But you will only be need of about 6 kg of Plutonium. Plutonium does not exist in the natur (exept for small traces), so it must be made in a reactor. The Plutonium is often alloyed with about 3 % Gallium.
Here is a drawing of the Plutonium bomb "Fat Man" which was dropped on Nagasaki.
The hard thing with this device is to get the explosive lenses to detonate exactly at the same time.
In both cases, the device is build to smash together a fissile material into a critical mass, with help of conventional explosives. Little Boy had a yield of about 15 kt, and Fat Man about 21 kt.