The most useful thing I was told about breastfeeding is that it can be very very hard at the start, but if you stick at it, it gets very easy. This was 100% true for me.
Make sure you eat and drink plenty yourself
Rest as much as you can (haha!)
Little one should have at least 2 dirty nappies (chicken korma consistency, size of a two pound coin) and 5 wet nappies a day, once your milk has come in. (Dirty nappies can slow down after a while which is fine)
Signs of dehydration are dry lips,dry eyes, sunken soft spot.
Don't be afraid to try different things to see what works for you. My little boy couldn't latch as he had an enormous tongue, a small mouth and my nipples were flat. He also had jaundice so was sleepy. I had to express and feed him with a bottle to wake him and then slowly went back to breast via nipple shields over a period of weeks as he learnt how to latch properly.
It will probably hurt but shouldn't be excruciating. If it is, something is likely wrong.
Be aware of tongue tie and seek assessment by a qualified practitioner if you suspect, do not accept GP/midwife reassurance as they generally are not qualified to diagnose.
Have a list of numbers to call if you need - NCT, national breastfeeding helpline, breastfeeding network
Research local support groups before the birth and go along if you need help afterwards
Kelly mom is a great website for info and reassurance about cluster feeding
Facebook also has some useful groups for breastfeeding support and a group called 'Can I breastfeed in it?' which is great for bf friendly clothing recommendations and just generally making you feel like what you are doing is normal.
Educate yourself so that when family members and friends wheel out the usual phrases 'little one can't be hungry again', ' you've only just fed', 'feed them four hourly', 'top them up' etc that you feel confident enough not to listen!