First, open your enlarging lens all the way (maximum aperture). Do your critical focusing this way using a focusing aid if available. Then, start with an aperture somewhat closed down from wide-open. That will increase the sharpness of your image (don't print with your lens wide open).
It would be good if you have an enlarging print scale which will allow you to get close to the proper exposure in one test exposure. It depends on the paper being used, but with such a scale, start with a 60 second exposure at say f8. If all your scale results are too light, drop your f-stop to f5.6. If everything is burned out, cut the exposure in half by increasing your f-stop to f11. You should be able to find an exposure where a good looking exposure scale results with one sector showing a good image tonal range. Use that sector factor to determine the actual time needed without the scale. Once you get close, it's probably best to leave the lens aperture at some set value and play with exposure time unless you find the time is impractically short or long - then you can resort to changing the lens aperture. Smaller (higher f-stop) means a lighter print. Larger (lower f-stop) means a darker print (these assume you're printing negatives and not making positive prints from slides).
It will take a little while and some work to get a feeling for the negatives and the paper you're working with. Be sure to write down the exposure time and aperture when you begin getting good prints and use this as a future starting point. There will almost always be some fine tuning required from shot to shot. If you change papers, the last usable exposure can be a good place to start with the exposure scale to find out how that new paper will behave (also refer to paper speeds if they're listed).
The other thing that can be helpful is an enlarging meter. This takes a reading of a blurred image and approximates the exposure required. It also allows you to repeat an exposure very accurately. You can certainly get good prints without one, but it takes more work and trial-and-error. You can accomplish the same thing as an exposure scale if you start a test print and sequentially cover (or uncover) portions of the paper at known intervals, noting the times used for each section, but that too, can result in more work. When you've mastered B&W,
may want to move on to color but there are more variables to control with color and the materials are considerably more costly. At that point, it would be wise to invest in an enlarger exposure meter so you don't need to waste paper finding the proper exposure (it can also involve a color analyzer to get the correct color).
Finally, the best exposure is the one which gives the most pleasing image to you (or your customer if your selling your work). There are many combinations of time and aperture which will do essentially the same thing so it's best to settle on changing one and leaving the other alone until you find a good looking image, but after you find the best exposure, you'll get to doing better in no time and get a sense of what to use.
Hope this helps - good printing!
https://www.ebay.com/itm/372209743613?chn=ps&_trkparms=ispr%3D1&amdata=enc%3...a6d51580293a5f http://www.darkroomautomation.com/em.htm