You need to first obtain an arrow shard from the Jotaro NPC (I think his name is the Marine Biologist) by pressing the "Stand help" button when speaking to him. Once you use it you have to stay alive for 10 minutes, and after that time you can get a stand. I think it should automatically equip to your stand slot, which is a dark blue tile at the bottom right of your inventory.

To actually summon the stand, you need to go into your controls and set a keybind. I don't know what the default is. Once you have that, simply press the key and you can summon/unsummon your stand.