The basic system is The Milton Work Count taught to all beginners. As we all know it’s A=4, K=3, Q=2, J=1. These are High Card Points (HCP). But this does not take account of distribution or ‘shape’ which might turn a dull, flat 10 hcp hand into an alluring one with a great deal of playing strength. So we can also add points for length and for shortage.
You should know that there are other systems of hand evaluation that take more account of distribution or side-suit high cards, or “losers” but for now we’ll stick with good old fashioned points.
A suit such as AKxxxx is worth 7HCP. But to this we can add + 2 LP (Length Points) , 1 LP per card above 4 in a suit. This is how you evaluate the hand before you first bid.
During the course of the bidding your hand can either increase or decrease in value as the auction progresses, mainly as a result of you and your partner discovering a fit or not. When you have discovered a fit you forget the LP you had initially and now instead add in your Shortage Points (SP) . With you and partner enjoying an 8-card or longer fit that will give a good trump suit, a void in a side suit is worth 5SP, a singleton = 3SP, and even a doubleton is worth a humble 1SP. So …
12 HCP's + 2 LP's = 14 — a reasonable opening hand.
A couple of rounds of bidding later, however, you find that you have a spade fit opposite and that partner has about 8 - 9 HCP. So now you re-evaluate your own hand in light of this information. The original 14+9 opposite is not quite enough for game, but let’s see what happens when we can add in the SP in place of the LP.
12 HCP + 4 SP = 16! Now 16 + 9 = 25 — enough to bid game easily.
The SP for the singleton heart and the doubleton club have increased the HCP of the hand by 33%.
There are other features of a hand that can make it worth more or less, but hopefully you will learn to trust your instinct on this, rather than hard math, as you play more.