Nettles in your lawn? Cutting it regularly before it gets knee height should deal with those. Anything with glyphosphate in it will do away with nettles at the root, but if there is a field full of them near you you will have to keep on dealing with them all the time I think. We have horsetail in one part of the garden and it's a total bugger to get rid of. I think I have managed it though. If you get horsetail put on a rubber glove and spray the weedkiller onto your hand and then gently crush the plant but not enough to destroy it, then the weedkiller gets down to the root system and kills it.
I quite like nettles though, I make nettle soup and nettle tea. I keep a few dandelions around because I like the leaves in salads and the tortoise likes to eat them as well.