You need a mixer to plug in the mics and the tablet. something simple like this mixer from Amazon that had enough inputs and has a basic echo effect for the vocals

https://smile.amazon.com/gp/product/B07HJ1CRQ5/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

You connect the output of the mixer into the input of your receiver. Depending on the karaoke software you probably connect the tablet to the mixer using a cable. Karaoke software like Karafun allows you to use Chromecast wirelessly but it requires additional hardware to connect the Chromecast to the mixer

You didn't mention the specific brand of receiver or speakers but I don't recommend using home audio equipment to sing through. Home audio equipment is not designed to handle the peaks generated when singing. To keep from possibly damaging your equipment you should use a powered PA speaker to sing through.