When you're beginning with magic it can go easily or it can be confusing. I'm seeing more and more confusion every day.
To begin with, when you cast a spell you shouldn't be able to tell whether it worked or not. Magic is typically considered to work along side the laws of nature. So in a nut shell, what wouldn't happen naturally, a spell would not do for you.
For instance someone wants to become a werewolf. If normally you would not turn into a raging furry monster with excess drool, a spell will not do it otherwise.
Another example is creating fireballs, not matter how many people claim that they typically make fireballs out of thin air simply at will, you need to understand they are delusional and a spell cannot make their legitimate.
So my first advice when choosing a spell to cast is to keep in mind the likelihood of the desired outcome actually coming to be. If chances are 1 in 23.5 billion, don't keep your hopes up.
Next I stress understanding. A lot of people will tell you to cast a successful spell you need passion and true desire for the desired outcome.
In truth, that's only one of several factors that goes into successful work.
Another one would be to understand what you're doing and how you're doing it.
To start off with I'll tell that magic in its most basic form is the constant changing and shifting of energy. To use magic takes some kind of method of directing energy around you to affect reality and hopefully bringing about change, that change being the desired outcome.
A spell is just a tool for directing energy. It's programming the energy around you to assist you in achieving your goal, or the desired outcome.
To cast a successful spell you should understand at least that much, and you need to think deeply about how it will all work, the series of events leading up to the desired outcome. You can't just ask the universe for a better job you need to know in what manner the universe will bring about a better job. You also need to put in your own effort. You can't move out into the remote desert and expect a job offer, you have to get out there and open yourself up to opportunities.
So my next piece of advice is to not just think about what's technically possible, you need to think about what you are willing to make possible. Life is not a box of chocolates that shows up at your front door automatically opening to show you what's inside. You have to save money, buy a box, find a pair of scissors, and open it yourself; a spell will help better your chances in finding that pair of scissors.