Overlay "ads"?

Hi I’m quite new to managing web pages with wordpress so bare with me if I ask some stupid questions here. Anyway..

I’m helping out with the website for a non-profit and I’m wondering how I can get one of those semi-transparent overlays to come up when you first visit the page to advertize recent events the organization has going on.. I’ve tried some googling but all I can seem to find is stuff that lets you sign up for a service where you get paid to display such ads, but you don’t get to choose them yourself.. I’m sure there’s a simple way to do it but I haven’t done ANY web development in a good 7-8 years so my skills are a bit rusty as is.. There any simple wp plugin or some sample code someone could help me out with?