Tl;dr starting with matt smith’s first series is fine. So is starting with Cristopher Ecclestons’s or Ncuti Gatwa’s, but you can start pretty much anywhere really.

I don’t know if you know this but doctor who is separated into two shows, “classic who” which was the original one and “nuWho” which is the current one. classic who is separated in 26 seasons, each one is divided in episodes, which are themselves separated into ~2-10 20 minutes parts. NuWho follows a more normal structure of series (and not seasons) divided into 45 min episodes. Finally the latest series (14) has technically been renamed to season 1 (because Disney got rights to the diffusion and wanted to rename it for newer audiences) but that’s confusing since classic who already used seasons so some people (including me) still call it series 14.

I wouldn’t necessarily recommend starting with classic who as the episodes are slower and longer (although if you like the older star trek shows it might be a similar rhythm (not sure as I’ve never watched star trek)) also a few of the episodes are missing.

If you’re starting with nuWho, I would recommend starting with series 1 with Christopher Eccleston, as this will allow you to learn the lore of the show without problems. Both Matt Smith’s Series 5 and Ncuti Gatwa’s Series 14 are also good starting points. Note that the latter is also the newest season and is meant to be a jumping point to the show.

With all that said you can still watch most episodes without any prior context, so start with whichever one you want, I would personally recommend “Vincent and the doctor”. Finally don’t listen to the fans too much when they tell you X doctor is bad and you shouldn’t watch them. Try watching what you want and try something else if you didn’t like it.