Where to start where to start! It very much depends on what you would like to do, what kind of travelling, budget, accommodation preferences etc. Many people choose to go to the north for their first time in India, and see Delhi, Taj Mahal (in Agra), some cities in Rajasthan eg. Jaipur, for example, perhaps Varanasi too. Creating a tour out of those locations would be fantastic, and you could do various types of accommodation eg. a mix of budget hotels, fancier hotels, desert tour etc. I don't know any details, but there are masses of tour companies who will arrange trips to this area, so you could easily do it without having to book things independently.
Equally, if you say you like the planning, then it's possible to put together your own trip, get a Lonely Planet (for better or for worse, they are a great source of info!) and consider doing a combo of train travel and private driver. Plenty of the trains in that region do daytime seat cars, but the night trains are fun too - I would recommend booking 'AC2' class (the AC meaning air conditioned, and the 2 meaning second class), as you will get your own bench / bed, plus bedding. The trains are hectic, but if you book AC2 they aren't as hectic as some of the scenes of Indian trains you see on TV :-)
I have travelled a lot in the mid to south of India, and if I were going back again, that's where I'd book to go. I loved the north, but for me there's something special about the south. You could, for example, fly into Mumbai, then take the Konkan Express train down to Goa - this is an absolutely beautiful journey. Goa has so much to offer, and is a really fun experience - my favourite place to stay is Arambol - aka Glastonbury on Sea! Near Arambol you have Mapusa market which is great for a look around, and you are not far from Panaji, Goa's capital, a beautiful town inspired with Portuguese architecture. From Goa you can take a trip (disclaimer: involving an overnight bus) to Hampi, and absolutely astonishing ancient archeological settlement - Google to find more details and pics, so hard to describe - but well worth the trip. The buses are quite hit and miss, and as we discovered, it is very easy to sell seats on smart looking buses with a nice glossy brochure, but the reality of the bus you get is often quite different :-) Not to say it's not fun, but worth knowing!!
From Goa you can head down towards Kerala, either by train or with a driver. Fort Cochin is a great place for tourists - a cute little harbour town with a good mix of hostels, hotels and fancy hotels. Some great food options there (including the fantastic Kashi Art Cafe) and some good western style food which can be welcome if you've been travelling in India for a while. From Cochin you can book houseboat tours on the Kerala backwaters - this is absolutely a must-do, you spend a few days just chilling out in a mixture of houseboats and paddle canoes going through the backwaters, and if you book a houseboat you get a cook and a guide, and you get beautiful fresh food cooked on your boat every day.
Kanyakumari (the tip of India) is popular, and it's quite fun to know you are on the furthest southern point of India, but I personally didn't like the place that much - wouldn't rule it out though as it does have some interesting museums etc.
Madurai, in Tamil Nadu, is a beautiful, hectic small city with the famous Sri Menakshi temple, absolutely amazing and well worth a visit. Madurai is a nice size and not too overwhelming, so good for exploring on foot. There are some decent reasonably priced hotels centrally, and loads of great food options. My favourite is College House - a complete mad canteen which looks highly unsalubrious but which does amazing South Indian food such as giant dosa and puri. Madurai has an amazing fabric and 'knick knack' market in one of the old temples which is fantastic fun, you can choose your fabric and get clothes made up the same day, and I love going there.
From Madurai you can train or fly to Bangalore, which I personally love. Although it's bigger than Madurai, it has a nice feel to it and can be explored on foot, or with an auto (rickshaw / tucktuck) if you get tired! It has Tippu Sultan's palace, some great markets and shops, and is quite interesting in the sense that it's a very modern feeling city with a Pizza Hut and a Hard Rock Cafe You can also reach Hampi from Bangalore, and also Mysore, which is again, very much worth a visit for its temples and markets.
As you can probably guess from my list, I've not done 'hardcore traveller travelling' ie complete backpacker roughing it, but I've done a mix of backpacking type, hotels and organised stuff. I also spent six months living and teaching in rural Tamil Nadu which was fantastic.
A lot of people have concerns at the moment about women's safety in India, and I would say it's well worth taking into consideration. I, for example, would feel happy enough doing train travel on my own, and getting rickshaws during the day in cities, although I would prefer to be travelling with someone. I would be less keen, very much less so, to do bus travel alone, especially overnight buses, and I would always try to avoid arriving places at night alone. Likewise I would personally not do any wildlifey off route stuff alone. Getting a private driver is not the exclusive tourist luxury it would be in some places - it can actually be quite a reasonably priced way to travel, and most good hotels will be able to arrange a driver. This comes with some potential safety considerations for a woman alone, and sensible measures like ensuring travel is mainly in the day, making sure no-one other than the driver accompanies etc, apply, but I would do it personally in some situations, not in others. If it were my very first time in India and I was travelling alone, I would consider booking a tour of some kind - as I say I don't know any companies in detail but I'm pretty sure there are tours which cater to all types of requests ie. backpacking right up to luxury.
My experience of India was that it is completely different to anything I'd ever experienced, and very intense and full on, but completely amazing. Be prepared for all the things you hear about (hassling and haggling with market stall owners, rickshaw drivers, guides etc), and accept that you will for sure get ripped off at times, but equally feel ready to enter into the spirit of the haggling etc :-) Read up and be aware of scams etc, but don't let them put you off! If you go with a 'expect the unexpected' mindset, you can't go far wrong.
Sorry this has turned out to be a bit of a stream of consciousness!! As you can probably guess I am a complete Indiaphile and will happily go on about it for ages and ages :-) But I'll stop here for now!