One of the main problems ectomorph's face is that in the early days the process of building muscle simply isn't fun.
It hurts, makes us feel sick, and the progress can be slow going.
Here are a few simple ideas you can try to actually enjoy the journey:

1. Set fun goals for yourself.
   Instead of simply pushing for more reps of any exercise, make it a game.
   Reward yourself when you hit X number of pull ups, or Y number of push ups.
   By doing this you're essentially changing your motivations from "building muscle" to much more enjoyable aim of beating your own personal bests.
   
2. Find exercises that work for you.
   Doing a "proper" routine of exercises is useless if you're not going to keep it up.
   It's better to follow a less effective or even unbalanced regime that you can actually keep up with for a long time.
   For example, although pull ups are the best ectomorphic exercise I certainly rely too heavily on them.
   But I justify it by realizing that I simply enjoy doing them.
   They're such a simple exercise, they look impressive to onlookers, and beating my own record is ultimately satisfying.
   Anyways, once you find your groove with a particular exercise you'll become fascinated with other ones to flesh out your routine.
   
3. Pump some music.
   Before doing any physical activity pump up some music on the loudest volume you can use without blowing out your ear drums.
   Upbeat rock music is a sure way to take the focus off the pain of exercise, and more on the fun, progressive side of it.
   
4. Challenge other people.
   Find people to workout with and then challenge each other to doing the most amount of reps, or lifting the heaviest amount of weight.
   It's amazing the strength you can uncover when your pride is at stake.