I want to create a world full of different civilisations and I want one in a desert but how would I make it believeable.
They would have had to start in a habitable zone and, after their water source dried up, had to rely on water farming and imports. They could also live beneath the earth where they have access to underground rivers, insects, fish and fungal growth.
No particular problems in making it believable. You can take as reference the cultures which developed on our planet and in our deserts, like
- Tuareg (for warm desert)
a large Berber ethnic confederation. They principally inhabit the Sahara desert, in a vast area stretching from far southwestern Libya to southern Algeria, Niger, Mali and Burkina Faso. Traditionally nomadic pastoralists, small groups of Tuareg are also found in northern Nigeria
- Sami (for cold desert)
an indigenous Finno-Ugric people inhabiting the Arctic area of Sápmi, which today encompasses parts of far northern Norway, Sweden, Finland, and the Kola Peninsula of Russia.
There are many more all along the history, so based on your world level and climate you can choose what suits best your needs.