You typically need a turbojet or turbofan engine on an aircraft to get that initial forward momentum before you can use a ram/scramjet. They both have a minimum speed required to operate which is achieved with a more typically jet engine.

All jet engines (as far as I know) require compressed air to create thrust. Think like the otto cycle of a four stroke engine, there is a compression stage where the air fuel mixture is compressed. Jet engines also require this compression, you'll see other comments mentioning "compressor stages" on turbojets - this is for the air compression.

The advantage of a scramjet is that supersonic air is already compressed, and therefore required no moving parts and can operate at higher speeds.

I am possibly wrong about some of this, I'm only an aero student sorry!