This goes on forever by the way. There are a few players out there who can flat pick and finger pick really well, but not many. Most players gravitate towards one or the other as their 'standard' mode. I chose my fingers. Fingerpicking Do me a favor .
Plectrums come in different sizes and thicknesses.
Tortoise shell was the traditional material but most are made of plastic nowadays. I know one great jazz player in Australia who uses wooden picks, apparently like the famous Django Reinhardt used to play with. One of the best ways to practice strumming and keeping time, is to play along to records of music you like. It will make it easier to fit the pieces of the puzzle together later on when you've got a collection. It's easier than you think.