Your Net Worth "is, quite simply, a snapshot of your overall financial situation, right now, in pounds and pence."
It's pretty simple. Just add up all your assets (I mean savings, bank account balance, cars, furniture, gold, property, returns from stocks, mutual funds, etc., everything, even the "£500 guitar lying neglected in the corner"), and subtract your liabilities (credit card liability, money borrowed, money owed to friends, etc., once again, everything there possibly could be).
The figure you thus obtain is your Net Worth. Just list the values of all your assets and subtract all liabilities. Calculating your net worth is that simple.
"In 2010, I worked out my net worth. The calculations were painless. What hurt was the number. I was £12,139 in the red. My debts were massive and I had no savings at all." (emphasis added)
Here's what you should consider about your Net Worth: it should never be negative. That would mean you owe more than you possess, which definitely isn't a very good proposition.
The idea is, always keep your Net Worth positive. And keep it as high positive as possible. And yes, do I even need to mention that it should increase every year?