Do I need a new deck to install subs & an amp?

Your head-unit ('deck') only powers your in-cabin speakers (typically tweeters and a few 6x9s). If you have subs and an amp, you should probably also buy a capacitor (1 farad) so that your headlights don't dim when the bass hits hard.

All the wiring will be provided by the installation place, assuming you go to a business that does these installations regularly.

only if your cd player doesnt have rca inputs and a turn on amp connection, bcuz you need those for the amp to work. either way but it depends bcuz sometimes its cheaper to get everything installed by them rather then to buy the equipment indiviually and have then install it.

it al depends if your stereo has the input at the back of it if so you wont have to buy new one and no you wont have to buy all the wiring b4 hand all stereo shops have it all. they will use the best stuff for what you want

its easier... straight up otherwise you have to get a in-line converter (i charge 30 bucks for one) but you can use that. it will sound about 50 times worse with factory radio in as well. tahts average difference.