Building a beat from one-shots
Most modern drums are programmed from one-shots: a kick on the pulse, a snare or clap on the backbeat, hats on the subdivisions, and percussion to add swing. Pick the kick first, because it sets the low end everything else has to fit around, then choose a snare that answers it. Audition sounds in a loop with the rest of the kit rather than solo, and keep a choke group between closed and open hats so they behave like a real hi-hat.
Acoustic, machine or both
Recorded acoustic drums bring natural dynamics and room sound; drum machine samples such as the TR-808, TR-909 and LinnDrum bring the punch and character of the records that made them famous. Many producers layer the two: an acoustic snare for body with a machine clap on top, or a sampled kick with an 808 underneath. Free drum machine banks vary in licence, so check each verdict.
Licences for drum samples
Drum one-shots are among the most openly licensed sounds online. Kenney's packs are CC0, many Freesound and Wikimedia Commons drums are CC0 or CC BY, and the vintage-machine samples on Free Wave Samples come with a royalty-free licence. Breaks lifted from old records are the exception: they belong to whoever owns the recording, so use breaks that their drummers licensed openly.