. deb files are for debian systems and shouldn’t really be installed on arch based distro, I think its possible but very difficult.
If you are new to manjaro then use the pamac gui (add and remove software option) to search for and install programs. If the deb file program isn’t in the official repos then its more than likely available in the AUR, flatpak or snap options in pamac.
Please, please, please don’t follow @stoner’s advice and follow @sawdoctor’s and just tell us what it is you want to install. We’ll help you!
@stoner I agree with @wongs that although your answer is 100% correct, it’s also 95% dangerous if @chump114 is new to Linux in general and 90% dangerous if he’s used to Linux but new to Arch-based distros…