Use maximum 2 fonts ,otherwise it will look messy. I would recommend you Montserrat and Lato fonts families.
Make sure that all elements align correctly .
In power point : view>grid
Google slides: right click>guide
Don’t try create color palette by yourself if you don’t know color theory(otherwise presentation will be ugly).Find color palette on pintrest or coolors.co.
Make it simple and clean without 3d forms,crazy animations and 50 objects on one slide.Minimalism always looks attractive.
Those four tips based on my experience as a presentation designer and they work in most cases.If you tried them and presentation still looks not good enough,you really need help of professional.