Can tile roof be installed over shingle?

No. You should rip the shingle roof off. Tile needs to have a smooth surface that is solid and sound. Shingles do not offer this. Depending on where you live you may want to add ice shields in addition to felt paper under the tile. Consult your local roofing supply company.

FYI, roofing is not technically hard, but it is quite physically demanding and going with tile, it's even heavier. You may be better off hiring someone for this and doing another project on your own.