Clair asks in the comments below:
I've never been to see anything remotely classical....what would you recommend as your favourite piece/person/composer etc. to introduce to new people.
Which I think is a bloody good question.
The simple answer would be to go to something with which you're familiar, perhaps from your record collection.
Years ago one friend asked another friend and myself how to start a collection of classical music. My recommendation was to get compilation discs. The purists would sniff, but sod the purists. The other friend set about to compile a list of definitive pieces. We argued. But the two of them got married so it became an irrelevant issue. They have since divorced, so I don't know where that leaves them.
Amazon are currently recommending 100 Popular Classics and The Only Classical Album You'll Ever Need, both at £8.99. You could do a lot worse than purchase these. The first has so many tracks I expect they're old recordings of obscure performers. No matter. The second has thirty one tracks, performed by a collection of top names.
I have various compilations, including Cinema Classics - there are 59 to choose from on Amazon; Classical Chillout, which are deliberately compiled to include pieces that people are familiar with from TV, film, adverts etc
I started building my collection haphazardly. Most of what my parents had when I was in my teens was on 78s, which is a tiresome format. I think while we were growing up there really wasn't the money to be spent on records, which were relatively expensive - a vinyl LP in the 80s cost in cash terms pretty much what you would pay for a non-chart CD now.
I developed my taste partly by being in school choir/orchestra etc and by doing music at O Level. So the first classical record I bought was Schumann's Kinderscenen because it was the first piece we studied on our syllabus.
As for concerts, the Guardian has a list of venues in London and outside
Just picking the Barbican's December programme, I don't think anybody can not like The Messiah - (although it clashes with Planning Committee).
Around Christmas, most concert halls put on programmes of popular or short pieces. They tend to be aimed at schoolies, so you have to put up with large quantities of children, but can be fun. I went to quite a few of these in my youth. The Mozart by Candlelight looks fun, but I shall probably be at Mother's then. However, the Bridgewater Hall has two excellent concerts that day, either of which I would recommend to a novice.
A lot of concerts try to combine a well-known piece, to draw people in, combined with something lesser known. My general advice would be not to spend too much money on going to see a star name - says me who's spending silly money to see/hear Murray Perahia in a programme of less than an hour's duration! This ticket is twice the price of last night's.
There are sometimes freebies available from the BBC
The other consideration is where to sit. I spent my entire youth in the cheap seats in the balcony at the Free Trade Hall. First my sister, then I, were members of the Halle Concerts Society, which meant that we got priority booking, and were rarely unable to get seats in the middle of the first or second row. This gave a wonderful view of the orchestra - I always have to see the percussion, my sister had a thing about conductors - and a fairly decent sound, although the FTH was criticised for its poor acoustics. The circle tends to be the most expensive.
Of course, this is just my inexpert view and very Brit-centred - although I'm sure the same principles apply in much of the Western world and beyond.