Cites are like that BECAUSE they are not designed for people but for cars.
Design your city to be nice and people will come. Once people come, crime will go down. Of you design a shithole then don't complain about the shit.
Start when? Tomorrow. Start how? Anywhere. Being with new construction requiring design for humans first. Make streets smaller and single direction. Build bicycle roads.
Oh also, stop the car manufacturer's lobbyists because they don't give a fuck that US cities are shit holes, they want to sell you more cars.
The Netherlands did t listen to them and see what it looks like now.