To create a new spreadsheet, visit a nonexistent file whose name ends with ".ses". For example, C-x C-f test.ses RET.
A cell identifier is a symbol with a column letter and a row number. Cell B7 is the 2nd column of the 7th row. For very wide spreadsheets, there are two column letters: cell AB7 is the 28th column of the 7th row. Super wide spreadsheets get AAA1, etc. You move around with the regular Emacs movement commands.
Point is always at the left edge of a cell, or at the empty endline. When mark is inactive, the current cell is underlined. When mark is active, the range is the highlighted rectangle of cells (SES always uses transient mark mode). Drag the mouse from A1 to A3 to create the range A1-A2. Many SES commands operate only on single cells, not ranges.