The first computers used punch cards or lights. Wikipedia says for ENIAC:
an IBM card punch was used for output
The German Wikipedia-article about Zuse's Z3 (the first apparatus that can be called computer) says:
einer Tastatur mit Lampenfeld für Ein- und Ausgabe von Zahlen und manuelle Steuerung von Berechnungen
That means: it had a keyboard with lights for input and output (programs were read from punch cards, as NobbZ said).
As computers got more common, they used often the already existent teletypes, that means output was made through printing text on paper. Until today, Unix-Terminal emulate to some degree teletypes.
EDIT: You asked specifically about PCs. First PCs came up in the 50s and had different methods for output. The IBM 610 used a keyboard for input and an electric typewriter for output. Also, some other early PCs used printers, i.e. Olivetti Programma 101. The Simon or the Kenbak-1 used lamps for output.
More infos about history of PCs at Wikipedia (as usual).
It's been a while since I spent much time there but it is very interesting and may be helpful. It will probably give you some computer names that you could then research to find out how people interacted with them.
To answer your specific questions:
- MARK I (1944): Used a 24 channel punched paper tape for input and output. Or for alternate manual data entry, 60 sets of 24 switches could be used.
- ENIAC (1946): Used a card reader for input, and a card punch for output.
- Simon (1950): Used a five hole tape for input and five lights for output.
- UNIVAC (1951) Used a keyboard for input and register lights for output.
- Xerox Alto (1973): Used mouse and keyboard for input and a Monitor for output.
- MITS Altair 8800 (1975): Used an ASCII keyboard for input and a '32 character alphanumeric display terminal' for output.
To illustrate an alternate input / output method in use near the dawn of the PC:
- Pong (1975): Used two knobs for input and a TV for output.
Some other early computers:
- Zuse Z1 (1938)
- Zuse Z2 (1939)
- Zuse Z3 (1941): Used punched 35 mm film stock.
- Atanasoff–Berry Computer (1942)
- Colossus Mark 1 & 2 (1944): Used patch cables and switches and paper tape for input and banks of lights for output.
- Zuse Z4 (1945): Used punched 35 mm film stock.
- Manchester Small-Scale Experimental Machine (Baby) (1948)
- Modified ENIAC (1948)
- EDSAC (1949)
- CSIRAC (1949)
And even before all that:
- Mechanical Calculator (Pascal) (1642)
- Jacquard Loom (1801)
- Scheutzian Calculation Engine (based on the design of the Difference Engine) (1843)
- Arithmometer (1851)
- Difference Engine (1859): As @artistoex pointed out, used a bell and a typewriter.
And even before that, as @WernerCD and @artistoex pointed out, calculation assistance has been going on for quite a while:
There have been many I/O devices in the history of computing, from simple light emitting devices such as incandescent light bulbs (and later LEDs) to punch cards to mechanically operated alpha-numerical displays. Pretty much any way you can think of interacting with anything has probably been used to interact with computers, such as bells, whistles and other such things.
Arrays of lights were probably the simplest form of monitor as they could be used in a similar way to the pixels you see on monitors these days.
Monitors are only one of the many ways that have been devised to interact with computers and it stuck because it is the most intuitive and technologically realistic method of dealing with information.
Since first Computer is known to be the Zuse Z1 (at least the first binary computer), I looked at it in wikipedia. But all I could find there was that the input was with punched tape. So I would guess, that the output comes on puched tapes too.
Probably you could find it interesting to go through that wikipedia-pages?
Depending on how early you mean, but printers are also output units. You punched in the program, and when it was done it printed the result.
Besides all the other output devices mentioned in otehr answers, TV sets were used for output by many computers in the 80s, like Spectrum, Commodore 64, etc.
The Difference Engine used brass digits, a bell and a typewriter.
The first "PC" related to what is commonly known as a PC today, the IBM 5150, used a monitor. You could obtain the system with a high-resolution text mode-only MDA card, or a lower-resolution text and/or color graphics CGA card. Around this time (1981) many home computers were designed to be connected to a television, and the CGA card could indeed be connected to a television.
The late 60's/early 70's saw the introduction of video terminals. These combined a keyboard, CRT display, and character generating hardware and were connected to a minicomputer or mainframe by way of some form of an RS-232 port. So for a while the "monitor" or terminal was considered part of separate self-contained peripheral.
The Altair 8800 had LEDs which you could use to show output if you did not connect a serial terminal to it.
Before the introduction of video terminals you had teletypes. Before then you had punch cards.