To start off with slayer, go to the Slayer Master (Duradel) by typing ::slayer, or by clicking ctrl+s on your keyboard. There are 4 difficulties of slayer task to select from, each providing different monsters you can fight.
Easy and Medium slayer tasks have no requirement.
Hard slayer tasks require that you've done one full prestige.
Boss slayer tasks require that you've done five full prestiges.
Easy Slayer Task Monster List:
Hoof (2,400 HP) [::hoof]
Crystal Skeleton (1,200 HP) [::ckey]
Undead Hero (20,000 HP) [::undead]
Mercenary Captain (8,000 HP) [::c