I have all my art supplies and I want to learn how to paint alla prima portraits on my summer break. what is the most efficient way to learn? any books i should buy or videos i should watch?

There are many short youtube videos for free on the internet, just google for "portrait ala prima demo" or something like that. One that is very nice is this one -

https://www.youtube.com/watch?v=qMv_nOHO_9g

Good to observe the brush size used in the initial phase. You wil get the best results with fairly art paint brushes indeed also when doing a bit more detail.