Welcome to the forum

Halfords can help, but I wouldn't trust them as far as I can throw them!
Checklist of things you'll need:
- New headunit/stereo
- Fascia adaptor
- Wiring harness (with steering wheel controls adaptor, if required)
- A basic knowledge of wiring
What year is your car? Whereabouts in the country are you? What's your budget? Will you just be changing the stereo, or are you thinking about upgrading the speakers and adding a sub too?
If you've seen something in Halfords that you like, make a note of the make and model and do a Google search - you'll almost definitely find it cheaper elsewhere!