During the first few days of Spanish class, most of us are taught a few basic rules to determine the gender of a noun. For example, nouns ending in "a" tend to be feminine, while nouns ending in "o" tend to be masculine. However, there are a few exceptions that tend to trip my students up.
The most common error I hear is "la problema." As a general rule, nouns ending in "ma" that are of Greek origin tend to be masculine. Of course, you may have a hard time determining the origin of words without looking them up, so I generally assume that all "ma" nouns are masculine unless told otherwise. Thus, you should say "el problema" and not "la problema." Similarly, you should say "el sistema" and not "la sistema."
Another common error I have noticed involves Spanish words starting with the letter "a." For example, agua and aula (classroom) both start with the letter "a." Although agua is actually a feminine word, saying "la agua" sounds incredibly weird. For this reason, you should say "el agua" and "el aula," even though agua and aula are actually feminine nouns. To make matters worse, since agua is feminine, make sure you use feminine adjectives to describe it. So, you should say "el agua clara" and not "el agua claro."
These are just two of the most common errors I have noticed while tutoring. Of course, there are a ton of other interesting masculinity and femininity rules I'd love to share! If you are interested, please contact me!