I think the best scale for starters is 15mm. Reaosnable prices, wide ranges available and reasonable tables. Said that I play in 1/300 (larger games with a lot of armor), 15mm (company-battalion) and 28mm (squad platoon).
20mm is a nice combination between 28mm and 15mm and 10mm is also a nice entry point (sometime I think I could have switched from 20mm to 10mm instead of 15mm). 10mm can also a good alternative for larger games if you do not want to sacrifice detail.
Arrigo