There are many types of fish preservation techniques.
1)Simplest one would be freezing. Either by just freezing the fish alone, or adding liquid/brine to the fish to be froze together. The frozen water/brine will then act as airtight protector to the fish.
2) curing
adding salt to the fish
3) drying/smoking
to reduce the moisture content of the fish
4) vacuum pack
pack in container/plastic bag without the presence of air
5) canning
There would be some other methods too other than what i've listed.
But it all depends on the type of fish, what would you like to do with the fish (way of consuming), and how long you would like it to last.