I'm 5ft 10" and weigh about 10 stone, I'm very slim but can't put on any weight.
That weight sounds fine for your height, but you probabyl suffer froma high metabolism.
The trick here is not to eat big quantities and lots of crap. Your body can only absorb so much in one sitting. To combat this, you wanna spread around 5-6 small meals throughout the day that your body can make use of, such as:
Breakfast -Shake in morning With cereal
10am - Fruit, cereal bar
1pm - Lunch, so a high protein meal if you can, soruces of tuna, boiled eggs, lean meat are stuff i went for.
3pm - Snack, maybe fruit again, won't hurt, perhaps another shake (no mroe than tow in one day)
6pm - main meal, again, get that protein inside you
9pm - light snack, bit of cheese, i like boiled eggs on their own, small salad, or the sake here instead of 3pm.
You will struggle with this at first, you won't be hungry for a couple of meals, you're body will be used to few, but it will adapt and it will get something from all of them. I can gain half a stone of muscle in 3-4 weeks this way (sounds bs, but providing you mix high protein witth heavy weights, it works).
Takes constant effort though to eat this amount.
Edit: in fact, last time i did it, when from 9st 8lbs to 10st 1lbs with no gain in fat, in like under 6 weeks and my shirts were very tight, now they are loose again
. That was only last autumn
. But i did weights 4 times a week for that, properly, else you jsut crap the protein out.
)