To add music/sound effects to your podcast, you will have to first find music that is copyright-free. The performing arts industry works on licensing, so be careful not to use someone's work without due credit to them, sometimes through attribution, other times monetarily.
To start off, we can simply use music that does not have copyrights. One such source we recommend is the ‘YouTube Audio Library’, which has a variety of amazing music and sound effects, and is always updated. You'll also find that they can be filtered according to genre, mood, instruments etc. to help you find the best piece of music that suits your podcast!