If I was arrested almost a month ago for 1.5 grams of marijuana, what can I expect when I go to court in a month and how much would it cost?

Start off by entering a general plea of "Not guilty."  Most judges will do this automatically for you when you appear.  The first time you are in court is usually for a "general announcement."  This is where the court tells you what the charges are officially, asks you for your plea (not guilty), and will usually inquire if you would like to talk to the prosecutor or hire an attorney. 

Because you sound like a youthful offender, I would recommend consulting and potentially hiring an attorney.  Many people enter pleas to misdemeanor charges with the attitude, "It's just a misdemeanor."  But it can still have consequences that may affect your education and eventual career goals.  For example, some scholarship boards will not continue funding you after a conviction.  If you are going to school for a particular license, some government agencies may not approve your application initially because of the conviction.  Basically, you need to be aware of all of the consequences of this charge before you make a final decision.  The consequences of a plea can and often do extend beyond the payment of a court fine.

As far as what will happen, if you are so young and you don't have a history, you may be able to participate in programs to "earn" a dismisal.  These will usually vary by jurisdiction-- so you always want to talk to an attorney that is familiar with your court.  They will be able to either ask or tell you what to ask for to get the lightest option possible.  If a dismissal is not possible, you may be elibile for programs which provide for expunctions.  This means that even if you are convicted, you could still get it removed from your record at a later date.  It's worth the effort to at least do a consultation with an attorney in your area to see what options your local court has to offer.  I also suggest talking to more than one.  This has the advantage of giving you a consistent feel of what will happen and second opinions tend to reveal different ideas and options.  I wish you the best of luck.