Depends on the fish. Some species simply spray their eggs and sperm into the ocean. Others make a nest and lay eggs, then choose a male to fertilise them. Some, like clownfish, defend a territory and wait for a long-term mate to win their approval.
It depends on the species, but most of the time males are trying to attract females. One way to do this is to by building a good nest and/or by being brightly coloured (in some species males have some control over their coloration because it depends on their diet). Males also do “courtship dances” to impress females. Another interesting thing is that females are more likely to mate with a male if they see other females mating with that male.
In some species, we actually observe sexual coercion, so there is no consent given to the male by the female, and instead the males force the females to mate with them (one example are sailfin mollies).
Shaylon and Natalie have given some good answers already so I’ll explain about how most fish reproduce.
Most fish don’t go to much effort to attract a mate (certainly not to extent lots of birds to) because most fish are broadcast spawners – females and males release clouds of sperm and eggs into the water and the fertilised eggs are carried away on the currents and eaten by loads of different animals, manta rays for instance. This strategy involves very little or no care from the parents but the vast numbers of offspring created means that a good number will survive. Adults don’t pair bond and because fertilisation is external, it’s likely that a single brood of eggs from a female will have multiple fathers
An adult Cod can lay as much as half a million eggs per kg of body weight a year and the record for the number of eggs per unit weight in a single female was estimated at 9 million from a 34kg fish!
Let’s say an average cod is about 10kg (they used to be much bigger but that’s overfishing’s fault). That means in a year, one cod will lay an average of 5 million eggs. A single population of cod might be a quarter of million adults, if not more – so that’s a potential larvae production of 1250 billion a year (that’s more than 10x more than all the humans who’ve ever lived on the earth). It’s likely that recruitment (the number of those that make it to adulthood) is a tiny % (much less than 1%) but even if only 1 in 100,000 larvae survive to adulthood, that’s still a potential recruitment of 12,500,000
This is how cod and other fish that reproduce in this way are able to create such massive populations. For mammals that produce only a few young a year at most it would take thousands of years to get anywhere close to cod but because we invest a lot of time and energy in caring for them, our survival rate is thousands of times higher