This post mostly repeats what others have said, but is more systematic and detailed.

The OP asked about aperture for the flash which sounds like an Automatic universal flash
of some sort, but I can't really be sure since he wasn't specific. So I'll just cover all the bases.

There are several kinds of on-camera flash units:

I. Universal --works with any camera (subject to voltage limitaitons!)
Only 1 connector in hot shoe: the trigger.
Dumb: -- no controls
Variable: -- allows you to vary the power
Smart -- contains a photocell, quenches flash after a predetermined amount of light
has been reflected from the subject. These unis sometimes are marked "Thyristor Controlled".

II. Dedicated: first only selected make/models, multiple conductors in hot shoe
Dumb -- like the above, but supports camera feature(s) (e.g., READY light in viewfinder)
Variable -- ditto
Smart -- ditto
TTL -- uses your camera's light meter. Camera controls the flash.

How you use the flash depends on what kind it is:

Dumb -- You need to know the flash units Guide Number. Guide numbers have
a unit: either feet or metes (usually both Guide Numbers are given).
1. Look up the flash's guide number in your favorite unit of measure (feet or meters).
E,g, 80 FT
2. Measure or estimate the distance to your subject in this unit. E.g 10 ft
3. Divide the guidenumber by the distancce: e.g., 80 ft / 10 ft = 8
4. Set this aperature on your camera. E.g., f/8
5. Set your camera's shutter speed to its maximum synch speed. E.g. 60
(Or slower, if you want more ambient light and slight overexposure).
6. Take the shot.

Variable -- Has a maximum power Guide Number. Compute as above. If you don't
like the aperture, you can reduce the power (and the Guide Number) and uses a
larger aperature.

Automatic -- These usually support Manual mode (proceed as with Dumb, above)
and at least one Automatic mode. Usually there are different automatic modes
for diffrent distance ranges: A1, A2, etc.

1. Measure or estimate the distance from flash to subject subject in your favorite
unit (feet or meters). E.g. 10 ft.
2. Enter your ISO film speed or digital ISO speed setting into the flash unit.
E.g., ISO 100.
3. Select the the Automatic mode that includes this distances. Eg. A2: 3-15 ft.
4. It will display an f-number. E.g., f/4.
5. Correct this number by total filter factor of any filters that are screwed to the lens.
6. Set this f/stop on your lens aperture ring (or however your cameras does it).
7. Take the shot.

The Automatic flash will measure the total amount of light that has been reflected
from the subject (and room) back at the flash. When this matches the predtermined
threshold, it will quench the Xenon tube, shutting off the light. If the subject and
reoom are "average" reflectance (tones), this will be a "normal" exposure.

Some Automatic flashes have a button that allows you to take a test flash, and an
indicator that lights up if the scene received enough exposure. If it didn't you
will need to move closer to your subject. This is very handy when shooting film.

The Vivitar 385 automatic flash supported an accessory cable taht allowed the
light sensor to remain on the camer while the flash was moved to a handle or
light stand. This was extremely useful.

Finally, if you have a flash meter, then it can be great assistnace when using any
Dumb, Variable or AUtomatic flash.

Dedicated Flash -- same as Automatic flash, with additional features.

TTL Dedicated Flash -- follow flash manufactuer's instructions. The camera
knows the ISO speed, so you don't have to enter it. Since the metering is TTL,
filters will automatically be accounted for. There are two DIFFERENT methods
of operation (sometimes you have a choice):
Method one: test flash of known brightness used to measure scene reflectivity
Method two: measure as you go (like an Automatic flash, but using the camera's meter)

Some cameras using method one flash up to three times to take one exposure:
1. Pre-flash (to set exposure and white balance)
2. Red-eye reduction flash
3. Exposure flash
This happens so quickly that it looks like one flash to the naked eye.

These flashes have far less power than they would if they only made one burst.
Also, opitcal triggers (to control slave flashes) cannot be used with flash systems
that flash more than one time per exposure.

TTL dedicated flashes are characterized by their high cost and complexity, but
are convenient to use and eliminate some (not all) of the guesswork.

It's not a bad idea to label flash units (of all kinds) with their trigger voltage and
full-power guide number: e.g., 150 VDC 120 FT.

My Bad! It's an unmitigated waste of time and effort to attempt to answer a specific technical question without the OP supplying all the pertinent information regarding the equipment in question, its specifications and the intended usage. Instead of all the well meaning folks around here being able to supply a logical, practical and on-point answer, everyone ends up engaging in a game of "20 questions", some post in-depth answers based on assumptions rather than facts that shoud have been supplied by the OP and of course as usual, infighting and name calling ensues.

Then, there is the usual outpouring of completely inaccurate information, especially where electronic flash is concerned. I am not talking about different approaches or diversity of opinion- I mean data that is diametrically opposite to the truth! Of course, there is always is enough good information that an astute reader could process, piece together like a puzzle and manage to filter out all the nonsense. In these scenarios, the OP usually wanders off into the the sunset- probably gets a good laugh out of the chaos and seldom reciprocates with so much as a "thank you".

My bad is joining into theses threads. Folks who actually know what the are talking about and sincerely want to help others troubleshoot and learn about photography shoud direct their efforts to questions that have sensible,logical and discernible content. I'll try to to better.