weigh ins...

However you do it, consistency is key. Most people find the best way to weigh under the same conditions each time is to do so first thing in the morning, right after using the bathroom. Nekkid, in skivvies or pj's is fine, as long as it's the same each time.

As for frequency - depends on whether or not you can handle seeing daily fluctuations up and down, because those are completely normal. If you're not good with that, then once a week or every two weeks is fine. Even still, you want to watch the overall trend, and realize that the scale will show a gain every now and then due to various things (water retention, hormone fluctuations, etc).

It's also a good idea to grab a tape measure and start tracking your measurements at least monthly as well.

In the morning
Naked or close (be consistent)
Before any intake
After using the bathroom

For how often I highly recommend every day or every other day.
Why?

When you weigh everyday, as long as you understand that EVERY SINGLE PERSON has weight fluctuations, you won't be disappointed by a small loss like you would if you do it once a week. You'll also be able to tell sooner if you actually aren't losing weight as compared to people who weigh once a month or even less than that.

Please understand that your weight will not go straight down. It'll jump up, jump down, back up and down again. The key is to keep it trending down and I feel weighing every day is the best way to see that.