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> Do I have to use solvent to get fine details?

You want to use a medium. You can use just a solvent - either odorless mineral spirits or pure turpentine - but only for a first, thin layer, like if you're using paint to draw in your scene or block in big areas of color or value as an underpainting. This has the advantage of drying very quickly but will also be very matte and dull; using solvent alone breaks up the oil so you don't get a good paint film on the dried surface.

A basic medium you can use for subsequent layers is half oil - linseed oil is the standard, and is the same stuff that's probably in your paints, but walnut oil also works and dries a bit more slowly than linseed - and half solvent. But there are a lot of other mediums out there; Liquin from Windsor-Newton or Galkyd from Gamblin (they're both basically the same thing, just different brands) are good all-around mediums.

There's a lot of different mediums out there that do different things, and you can experiment with them as you get more experienced, but any of those I mentioned are perfectly good and a lot of artists go their whole careers only ever using just one of those.

Edit: Also, read this short article on the basic rules for layering your paints; it's essential knowledge if you're going to be using oil paints.

You can also totally oil paint on pretty much any surface if you're just practicing or doing little quick studies, but you can pick up big packs of canvas panels pretty inexpensively on Amazon. The five paintings pinned above my drying shelf here are on either cardboard or bristol board that I glued down to cardboard (basically homemade illustration board); the ones at the bottom actually leaning are in-progress pieces on canvas panels. I also picked up some hardboard panels to work on, and they're not much more expensive than the canvas ones.