I've been wondering how to do this for a while but I couldn't find anything that gave me answers. I'm trying to make a realistic-ish model but I don't know how to round the edges.
You can use the Bevel tool.
- In Edit mode, select all the edges you want to be rounded.
- Hit Ctrl+Shift+B to initiate the tool.
- Adjust the offset (radius) of the rounded corners by sliding your mouse.
- Adjust the number of segments (subdivisions) you want with the mouse wheel. The more there is, the smoother the corner will be.
- Validate the operation with a mouse click or by pressing Enter.
You can adjust those parameters, and many others, by expanding the "Last Operator" panel (bottom left of the Viewport; it should be labelled "Bevel"):
I assume you want to round edges of a rectangle, a planar surface; if you want to round the edges of a volume, you should use Ctrl+B instead.