I am new to Premiere and have a problem, I am working on a project with stills that are in 4:3 and videos that are in 16:9, does anyone know how I make it work? Would be really thankful for tips.
First, a video can only have one aspect ratio ... you can't change it mid-vid. So you have to pick which aspect ratio you want for the job, and then, what to do with the bits that don't fit.
The typical choice these days as most screen handle 16:9 better than 4:3 is to use a 16:9 AR in the timeline.
Then choose whether you want to show the whole 4:3 image, which will require of course the black bars on either side, or zooming in on the 4:3 image to fill the 16:9 AR.
In the Preferences there are options for adding non-to--scale bits, Set to framesize and Scale to framesize. If you set one of those, use Set to Framesize ... do NOT use 'scale to'.
You can also use a color matte or other background on a video track below the stills instead of having black bars if you use a 16:9 timeline.
Another option is to duplicate the still on a video track below the still and increase the scale and/or add a blur to the duplicated clip.
Most likely your export format will be 16:9, so you'll need to zoom in your still photos to fit within the 16:9 frame, or you can create/use a template to put an oddly sized image into a frame that makes it look nice in a 16:9 timeline, like this: