To insert printing characters into the text you are editing, just type them. Emacs will automatically insert the characters that you type into the buffer at the cursor. The cursor moves forward, but if you prefer to have text characters replace (overwrite) existing text characters, you can enable the Overstrike option from the Options menu in the menu bar.
To delete text you have just inserted, use DEL. DEL deletes the character before the cursor (not the one that the cursor is on top of or under; that is the character after the cursor). The cursor and all characters after it move backwards. Therefore, if you type a printing character and then type DEL, they cancel out.
To end a line and start typing a new one, type RET. This inserts a newline character in the buffer. If point is in the middle of a line, RET splits the line. Typing DEL when the cursor is at the beginning of a line rubs out the newline before the line, thus joining the line with the preceding line.
Emacs automatically splits lines when they become too long, if you turn on a special mode called Auto Fill mode. See Filling in SXEmacs User’s Manual, for information on using Auto Fill mode.