The following rules of word division apply to most words you're likely to use:
No syllable starts with more than one consonant unit: A "consonant unit" for the purposes of word division is either a single consonant (as in English, typically any letter other than a, e, i, o and u) or one of the following combinations, which (except in rare exceptions) are not separated:
Note how these rules apply to the following words: ga-to, si-la-bi-fi-ca-ción, pe-rro, in-cre-í-ble, par-te, glo-sa-rio, chis-te, ca-lle, tri-ple, flo-ri-da, com-pra-do, por-que, ex-pli-car, in-no-va-ción, in-gles, fe-cha.
Diphthongs are not divided: As a practical matter, this rule means that a u or an i is not separated from an adjacent vowel unless the u or i is accented and is next to a "strong" vowel (a, e or o). Other adjacent vowels can be separated.
See how this rule applies in the following words: fui-mos, bue-no, ciu-dad, cre-er, puer-to, fe-os, fe-as, plei-to, des-pier-ta, ha-bí-an, rí-os, u-to-pí-as.
Single letters avoided: Unless you're dealing with very narrow columns of type, you should not leave a single letter standing by itself on one line. Thus, while you may divide ríos, you should avoid dividing río.
Keep prefixes intact: You may break the first rule above to keep a prefix intact. Thus, sub-ur-bio, not su-bur-bio, and des-or-den, not de-sor-den.
The rr exception: When a prefix ending in a vowel precedes a word that begins with an r, the r typically doubled, as in prorrumpir and contrarrevolución. When such words are divided after the prefix, the rr on the following line reverts to a single r. For example, if prorrumpir is divided after the prefix, it becomes pro-rumpir, not pro-rrumpir, and if contrarrevolución is divided, it becomes contra-revolución.