How do you know in spanish when to say un or una before a word?

If the Word is masculine, you say un (unos when plural). If the Word is feminine, you say una (unaswhen plural).

As general guidlines:

• Spanish Words that end in -o,-e,-on are usually masculine. There are some exceptions to this such as clase(class), which is feminine.

• Spanish Words that end in -a are usually feminine. Again there are exceptions to this rule.