Hi nieves! Welcome!
I can already imagine some of the replies that you might get this this question so I'll jump right in with my tips. Despite what my post count might tell you, I'm not that new to this board (just finally crawling out of my shell enough to post) and I'm not an entire newbie to mystery shopping. However, I don't have anywhere near the experience that some of the members on this board have. Here goes:
* Don't kid yourself into thinking that you will make $3000/month from mystery shopping right away. Probably won't happen. In fact, for awhile, you may not even come close to that number.
* Learn everything you can possibly learn from the veteran members on this board. Some of them probably do make $3000/month and maybe more. Maybe a lot more. But I'm sure they'll tell you that it wasn't like that at first.
* Sign up with as many mystery shopping companies as you possibly can. There's a blue link at the bottom of the forum page that says "Official List of Mystery Shopping Companies". Use it. Set aside time each day to sign up with a few each day. You may be surprised to find that most of the successful shoppers who make a fulltime living as a MS'er are signed up with upwards of 200-300 mystery shopping companies. They don't all have shops in your area right now but they might one day have a shop. You won't know if you don't sign up.
* Expect and Accept the fact that as a new shopper (and I'm making the assumption that you are new to Mystery Shopping) you will not get the highest paying jobs, the best and most exciting jobs, or your pick of assignments until you "pay your dues" and establish solid & credible histories with each of these companies. The only way you can do that is by accepting assignments at less than you might want to take or assignments that you're really not all that interested in and giving those assignments your all. Treat each and every assignment with the kind of detail and attention that you would give if it were paying you that $3000 you want. That's the only way you'll get to where you want to get. The best and highest paid assignments are often not advertised and are (from everything I've experienced and learned on this board) held for those shoppers who have proven themselves and established an excellent track record. One MSC (I believe it's Coyle Hospitality) even states on their website that if you want the exciting and flashy cruise & hotel assignments, you must prove yourself first by accepting some of their "lesser" (i.e. less paying, less "exciting", or less responsibility) dining shops first. They reserve their best of the best FOR their best of the best.
* Remember that every shop you do and every person that you come into contact with, whether it be a scheduler or another shopper, is an opportunity to build a relationship. You must conduct yourself professionally and respectfully at all times. If you badmouth a scheduler or an MSC on this board, a representative from that company may read that post. At the very least, representitives from other companies will read that post and your reputation will be affected. Same goes with the way that you conduct yourself toward other shoppers.
* If your writing, narrative, or spelling skills leave something to be desired then you need to focus on improvement. If you don't have a mind for detail then you need to come up with a system for ensuring that you remember the pertinent details of every shop. Don't have a firm grasp on following directions? Figure out how to make sure that you fully understand everything being asked of you. It blows my mind how many people think that they can be successful at mystery shopping without knowing how to read, write, and remember. It doesn't mean that you can't be successful if your writing and grammar skills aren't top notch right now or if you can't remember what you had for breakfast by 10am let alone 40 pages of shop instruction. It means that you have to have a firm understanding that you MUST improve those skills in order to be successful. You will not, under any circumstances, be successful as a shopper if you cannot improve along the way.
I wish you luck and feel free to ask questions! I've found that most people on this board are willing to help if you're willing to accept the feedback.