I start with Sculptris for most stuff. It's a free modelling program and it's actually designed around the idea of modelling the stuff like if you were modelling something out of clay so it's really effective for the artstyle I want - a key to game design is use our limits to our advantage. I've chosen an artstyle that doesn't require much knowledge (if you can sculpt something out of clay you can do it), that keeps to the relatively limited look of old SNES games and that I can do when being one person (if I had a team, I'd probably have chosen a more complicated artstyle but the man hours required for triple A games is impossible for one person).
Basically in Sculptris we start with either a flat plane which we can "draw" on top of using the mouse, or we start with a sphere which we can mould into the desired shape using the mouse.
I export the model into Blender, exporting each moveable part separately (eg the head, torso, arms, hands, legs, feet) just so it's easier to edit separately.
Then I use Blender to "dissect" it (ie the parts that were built on the same sphere, like how the hair and nose are both just drawn into Ness' face sphere). Then I export each part as a mesh, into Roblox Studio, where I add colour, lighting etc. The biggest problem with Studio is that we can only import a mesh that's less than 10,000 triangles. Sculptris moulds everything using a lot of triangles. I can reduce the number of triangles in Blender but if I reduce it by too much it can't be read in Studio. Ness himself is at least 200,000 triangles, meaning I had to break him up into the smallest chunks and piece him back together in Studio. Likewise Studio can only assign one colour to a mesh, so each different colour you see in a model (eg the stripes on his t-shirt) are their own mesh - I haven't used textures, except for the grass/wood textures that are built into Studio, so every colour, design etc is a mesh instead of a flat image.
It's an easy modelling process but it's time consuming!
That said, stuff that can be made easily in Studio I made them in that (the house, fencing and mailbox are all made in Studio, everything else is done the Sculptris way)
Sculptris, Blender and Studio are all free to use and require little hardware specs so I'd recommend any/all of them. That said Roblox has a cringeworthy online community of mainly kids so I'd advise not to use the online features, just Studio itself. You can always export it to another game engine afterwards if you desire but Roblox is good if you want to have a quickly viewable level that you don't need to program for (eg when making fan recreations of videogame maps)
Also, check out The Models Resource. If there's a 3D model in a videogame, chances are it's been uploaded to that. For characters, I download the model and view it in Blender while modelling my version in Sculptris, so I can keep it as accurate as possible. That's only if you want to make fan projects though, and if you wanted to be really lazy you could just use those models themselves!