Circuits are defined by a text netlist. The netlist consists of a list of circuit elements and their nodes, model definitions, and other SPICE commands.
The netlist is usually graphically entered. To start a new schematic, select the File=>Open menu item. A windows file browser will appear. Either select an existing schematic and save it under a new name or type in a new name to create a new blank schematic file. LTspice uses many different types of files and documents. You will want to make a file with a file name extension of ".asc". The schematic capture commands are under the Edit menu. Keyboard shortcuts for the commands are listed under Schematic Editor Overview.
When you simulate a schematic, the netlist information is extracted from the schematic graphical information to a file with the same name as the schematic but with a file extension of ".net". LTspice reads in this netlist.
You can also open, simulate, and edit a text netlist generated either by hand or externally generated. Files with the extensions ".net", ".cir", or ".sp" are recognized by LTspice as netlists.
This section of the help documents the syntax used in netlists, but occasionally gives schematic-level advice.