Hi there Chrissy -
As long as you weigh yourself around about the same time, once a week or even once a fortnight is fine for how often you should weigh in (any more regular than that and you'll do your head in - our bodies fluctuate so much through a single day that you could go quite spare keeping track of it - not to mention how upsetting it can be if you see a minor gain, or a false sense of security if you see a loss...)
As for measuring, once a month should be fine, as it's spaced enough to begin seeing changes. If you really want to measure weekly, that's fine, but it's usually such a fiddly little operation that you mightn't be bothered doing it so often. You might like to keep an eye on one specific area, eg I measure my waist weekly, but only track it monthly.
For an all over measure, keep track of these: neck, chest, each arm (around the bicep), waist, hips, each thigh, each calf. You might like to try and figure out a way best to remind yourself of where you are measuring - it's such a specific skill, that it's a whole entire subject at some of the more reputable sports and personal training schools.
Hope that helps.