Where to start... !
A Reception maths lesson may take many forms! However- it does not look anything like most maths lessons in statutory school-age classes. They take place in small groups and run over from session to session. Sometimes some take place for very short periods on the carpet as a whole class. Neither should maths be planned a week at a time, as you have to build on what children know and can do, so 2 or 3 days ahead is good, and then a re-think. R teachers do not have to use the planning framework in use in the rest of the school.
I would start by visiting the R class whilst they are at work/play- quite regularly and discussing with the practitioners how they introduce maths to their children, what they do and the questions they ask to extend chidlren's thinkng. Look at the resources they have. Go in with the spirit of learning from them! Ask them what their concerns and needs are.
Yoiu should read the EYFS guidance on Early Yrs maths and look at the Early Yrs section of the Nrich site: http://nrich.maths.org/early-years - great ideas and exactly what a 'lesson' should look like.
If you want anything to read - post again! Sue Gifford is excellent.
Good luck and enjoy it - it's the most exciting phase!
One more point. Beware of top-down policies putting inapproporiate pressure on R children.