El leísmo: In some parts of Spain, le and les are used as direct-object pronouns to refer to masculine human beings instead of lo and los, respectively. You're not likely to run into this usage, known as el leísmo, in Latin America.
Se: To avoid alliteration, when le or les as an indirect-object pronoun precedes the direct-object pronoun lo, los, la or las, se is used instead of le or les. Quiero dárselo, I want to give it to him (or her or you). Se lo daré, I will give it to him (or her or you).
Placement of object pronouns after verbs: Object pronouns are placed after infinitives (the unconjugated form of the verb that ends in -ar, -er or -ir), gerunds (the form of the verb that ends in -ando or -endo, generally equivalent to the "-ing" ending in English), and the affirmative imperative. Quiero abrirla, I want to open it. No estoy abriéndola, I am not opening it. Ábrela, open it. Note that where the pronunciation requires it, an orthographic accent needs to be added to the verb.
Placement of object pronouns before verbs: Object pronouns are placed before verb forms except those listed above, in other words, after nearly all the conjugated forms. Quiero que la abras, I want you to open is. No la abro, I am not opening it. No la abras, don't open it.
Order of object pronouns: When both direct-object and indirect-object pronouns are objects of the same verb, the indirect object comes before the direct object. Me lo dará, he will give it to me. Quiero dártelo, I want to give it to you.
Obviously, there are quite a few rules to learn! But you'll find that as you read and listen to Spanish that the rules will become a natural part of your understanding of the language.
