As regards to the sentences, the nouns are guitarra y fiesta (first sentence) and amiga (second sentence). In the first sentence the subject is Juan, whereas Yo is the subject in the second sentence.
For the definite and indefinite articles, the same rule applies in Spanish and in English. If we are referring to something in particular, we use the definite article (English equivalent THE). If this is not the case, use the indefinite article (English equivalent A). When you have decided upon which article to use, then proceed to the next step which is determining whether the article has to be singular or plural, masculine or feminine.
As for adjectives, they come right after the noun, not like in English. For example, if we say A TALL BOY, in Spanish this would be UN CHICO ALTO (alto is the adjective). We only write adjectives of quantity and sometimes of quality. Adjectives of quantity : POCO dinero (it's telling us how much money). Adjectives of quality : when something is inherent (it has to be like that, there cannot be another way) la BLANCA nieve (snow is white, it cannot be of another colour) The difference between UN CHICO ALTO and LA BLANCA NIEVE is that in the first example, that boy happened to be tall, it doesn't mean that all boys are tall. In the second example, snow is white, it cannot be of any other colour.
The explanation for the various uses of ser may be found : [link]
Hope I've been of some help. If you need to clarify about something, I'll be here together with other Spanish lovers 