No 4 to start to get a feeling about using a plane. If you’re like me and start with small boxes, that should get you going.

Next a no 5 and possibly another no 4 really cheap to turn into a scrub plane. That really got me going on rough lumber.
Just a few days ago I made some handles out of a log of firewood. Last year the same thing with only a no 4 took me several hours, now I got my blanks foursquare in under 40 mins using my scrub and a smoother.

I went for the smaller version of the ezelap diamond stones. Do get some good chromeoxide polishing compound, it made stropping a hell of a lot easier. I got mine from ‘bay.

Just my opinion.
HTH
Diego