Hi and welcome to the forum abijchen. I could be wrong, but I don't think anyone is doing guided walks to Cradle Summit. For an inexperienced walker, it might be quite challenging. You have to scramble up some very steep sections, and heave yourself over some hefty rocks. That said, I've seen walkers from 8 to 80 complete the walk. If you're feeling uncertain about tackling it, I'd suggest starting with one of the less challenging options (eg Marions Lookout, Crater Lake Circuit or Dove Lake Circuit) and see how you go. Also see if you can find any others going to the summit who'd let you tag along. There are plenty of friendly walkers around in summer.
A good place to get info. is the "60 Great Short Walks" section on the Parks & Wildlife website. A brief description of the Cradle Summit walk can be found here
http://www.parks.tas.gov.au/indeX.aspX?base=1362 Note there are hot links within that page that give a sketch map and other details. Go back a menu for other walks. If you have an iPhone, iPod Touch or iPad you can also get "60 Great Short Walks" as an app (free from the iTunes App Store). Four of the walks in the Cradle area are on this app/brochure.
I would also recommend that you take a map.The "Cradle Mountain Day Walk Map and Notes" is available at the park or from TASMAP online
https://www.tasmap.tas.gov.au/do/product/NATPARKDAYWALK/WKCRAD Of course weather is another important factor. Ask for a forecast and other info. when you're at the park, and tailor your walks to fit that. Whatever you do there, I reckon you'll have a great time! Enjoy
cheers
Peter