My computer crashed and discovered that the virus called Trojan, and the need to hire professionals to eliminate them and it cost me a lot. My question is how to prevent this from happening again, any tips greatly appreciated.

just install an anti-virus program. Myself I use the free version AntiVir.
Ans from time to time scan your pc for malware with for example malwarebytes wich is free as well.

Trojan Viruses can do a lot of harm to your computer or worse. There are plenty of free and paid anti-virus software out there that you can download free of cost. And update your anti virus application day by day.

Once a Trojan virus active in your system, you can ruin a greater chaos in the system by bringing down all the defence mechanisms of your system.