I need to upgrade my bike

[Reem the winer]

Well I got a vilano bike disk brake 7 speed free wheel quick release but the awful thin shifter so I am thing to change to shimnao calris 2x8 But the problem how I change the 7 speed gears to 8 gears Bruce’s it is free wheel not Cassatt system if change the hub to a Cassatt hub 8peed will it fits in my frame ????? Even if I choose tourney sti shifters system 2x7 do I need to change the crank set and add the shifter to sti. Or I need to change more stuff what is better calris system or tourney or they are near???? The front devil is 7 speed tourney Tz 7 speed the back one is tourney 6/7 speed both of them are sis index thanks