Scythes are a weapon with two main types, melee and magic.
The most common type of scythe is the magic type. Functioning almost exactly like a staff, it serves as just a way to focus your magic or to look cool as you cast it. It can double as a melee scythe as well, but scythes designed more for melee will nearly always beat a magic scythe in a melee fight.
A major problem for the magic scythes is their extra size. The blade is rarely actually used, except for certain scythes in which the metal may actually help them absorb or create extra magic energy. Sometimes, staffs/staves are chosen over scythes for this very reason. A quick way to tell which one you want is to ask yourself if there is any chance the enemies you will fight will get close to you. If yes, pick a scythe. As said earlier, a magic scythe can double as a somewhat deadly melee weapon, but a staff is pretty much a stick.
Melee scythes tend to be lightweight, quick, very sharp, and not nearly as succesful as a weapon that is actually designed for fighting other people/things at melee, not cutting grass .
Though, despite them not being as widely used and appreciated as the magic-style scythes, they have had some success against hammers, another two-handed weapon. Hammers aren't typically able to cut, but a scythe is designed to cut. Thus, one could slice someone with a scythe in a fight, and wait for them to bleed out or give up and leave thanks to pain and major injury. However, with a hammer, typically, the worst bleeding would be from bruising you'd cause, and there are tons upon tons of instant cures for that. Hammers can break bones, scythes can take limbs.
Scythes are rarely used outside of areas with lots of demon-like monsters, with the exception of Mourncrest. If you want to find lots of scythes with little cost, going to an uncivilized area known for demonic and magic enemies would be your best bet. If you aren't willing to fight, however, a trip to Mourncrest would be worth it. Though, fancy and good scythes in Mourncrest have been known to be expensive.