Clothing advice for beginner
I'd invest in a pair of bike shorts.

Casual riders going short distances on big comfy saddles can get away with casual or regular sports clothing. You won’t be able to do that riding significant distance on a sports saddle. 

Bike shorts have padding, obviously, but are also constructed so there is no seam between rider and saddle. Regular sports shorts have a central crotch seam that is painful to ride on. 

The reason to not wear underwear under bike shorts is to not re-introduce seams between rider and saddle.