Guwahati isn’t called the gateway to North East for nothing! Unless you’re already in Sikkim or Kolkata, the easiest and most convenient way is to fly.
There are daily direct flights to Guwahati from New Delhi, Jaipur, Mumbai, Bangalore, Chennai, Hyderabad and Kolkata. I flew with Spicejet direct from Jaipur, it took 2 hours and it was really cheap!
If you’re flying internationally (for example from UK) than you will stop over in Delhi before your onward flight.
You can get a direct train from Varanasi to Guwahati but it will take around 30 hours. I’m not sure if it’s actually much cheaper than flying but it’s up to you if you like the long road. See my helpful guide for booking train tickets in India here.
As I said, Guwahati transport links are vast and well connected as is most of Assam. Take advantage of the easy transport now before you get into the other states. The transport options are a challenge in those!
Government buses to Jorhat, Kaziranga National Park, Dimapur and Tezpur you can book from Paltan Bazaar.
If you’re heading to Shillong/Cherrapunjee via public transport, you can travel by Sumo! Sumo’s are essentially sharing jeeps. You wait for it to fill up and you head on out! They’re super cheap if you have the time to wait.
Public transport in North East come in all shapes and sizes. As well as jeeps, there’s smaller cars which fill up faster. You pay a little bit more, but not by much!