I would strongly advise against getting one of these moisture meters, they're not too accurate unless you splash out on a really good one, for seedlings as suggested above, i'd just poke your finger into the soil and just water very gently, and then once it has established itself you can simply water till there is runoff and just learn the difference between the weight of a wet and a dry pot . When watering plants as opposed to seedlings you will be given nothing in terms of advice but learning the weight of the pot. Again, a bit vague and hard to tell at first, but you rapidly learn and you'll wonder why it was such a hard thing to do