How do I know the gender of a new word?

The ending of the word gives you a strong indication of its gender, if it is a new noun. 

The most common of these endings are listed below:

If it ends in -é, -enne, -ette, -ande, ière, or -appe it will be feminine: une possibilité, une comedienne, une étiquette, une bande, une prière.

If it ends in i, -ont, -u, -at, -age, -ing it will be masculine: un ennui, un front, un inconnu, un paysage, un planning.

Of course, there are exceptions to every rule but when in doubt, you should be able to make an educated guess if you remember the rules.