Jethro Tull where to start?

I started with Songs from the Wood.

Aqualung is their best album, and although not necessarily "prog" in a conventional sense it does set up the next albums Thick as a Brick and A Passion Play nicely from a thematic and conceptual standpoint.

Aqualung is the best start then go to Thick As a Brick

I echo all these recommendations. But I would also urge you not to overlook War Child. Shorter tunes, but many gems there too. (e.g. Skating Away... is an awesome Tull song, and Sea Lion, etc)

Often enough I like to strat getting to know bands with Live albums, because that way I can know a good selection of (hopefully) their best songs, and if I'm interested enough to get their discography later on I won't be hesitant about having so much repeated songs as if I got a best of album instead. Jethro Tull is one of those bands in which I think this works wonderfully, so I would say "Live - Bursting Out" and then "Live at Montreux" (though there's lots of complaints about his diminished vocals by the time of Montreux, but it doesn't really bother me very much). In general, most of my favourite songs in my favourite versions come from live albums, particularly these two... I feel they got more power and were further developed to the point I find the studio ones incomplete or too tame. As for studio albums, yeah, it would have to be "Aqualung", "Thick as a Brick", and "Songs from the Wood".

I'll vote for Thick As A Brick (I always prefer jumping right into the thick of it, no pun intended). 

Junges wrote: If you start in the 60s or 70s, it doesn't really matter. If you don't like an album, try another. There is no science of "how to get into a band".

I really recommend to start with Songs From the Wood. Quite proggy, but easier to get in than for example Thick as a Brick. Also it already have some modern taste in it. Of course Thick as a Brick is their greatest album.

I heard Aqualung first and at the time it didn't tell me much (apart from the title track). Somehow I was too young for it (13 or 14) with a too restricted horizon, don't know. Now I appreciate it much more than I did in the beginning. So I may be an outlier but I wouldn't recommend that one to start. I still find it a bit disjointed, apart from some really great songs it features a number of styles that I don't connect to that well.
What got me into JT was Thick as a Brick, but also Heavy Horses, Stormwatch and even Broadsword and Beast had a more immediate appeal to me than Aqualung. I was looking for long complex compositions and instrumental flow at the time but as somebody who got into music in the early 80s I was also attracted to then "modern" keyboard sounds and some more compact but sharp songwriting. Obviously it depends on where you come from and what you're up for.

Thinking it over, I find that Dellinger's approach through live albums (here Bursting Out) makes a lot of sense, and it was a good path that brought me to some other bands.