Start it from season 5 (2005)

I mean season 1from 2005

yeah, I'd agree with that
I think there's 9 season now? in that time there've been 4 doctors
it's a pretty decent show, obviously not perfect or as good as some other things, but it is due some credit for being not-awful sci fi
give it a try, your username is thelostfandom so you'll probably enjoy it

but what is the seasons with David Tennant bit confusing trying to figure it

Ok, in case you don't know, every couple of seasons the Doctor "regenerates" and is replaced with a new actor. Season 1 (aired in 2005) had Christopher Eccleston as the Doctor, and at the end of that season he regenerated into David Tennant. Tennant was then Doctor from season 2 until the end of season 4, when he regenerated into Matt Smith. So, if you're only interested in the David Tennant episodes (my personal favourites) then watch season 2 (aired in April 2006), season 3 (March 2007), and season 4 (April 2008). From there, you might be interested in continuing the show, so feel free to keep going to season 5 or head back to season 1. And don't worry, the seasons are relatively self-contained, so there are very few times that you'll be confused due to lack of information.

And most importantly, have fun. Doctor Who is just fun to watch, so enjoy yourself :)
hope I helped

I recommend watching from season 1 (2005) tho, there's so much going on between the doctor and his companions sometime she/he will miss things