How does alternative splicing work?
21joanna12, look into snRNPs.  These are parts of the splicosomal apparatus and some of them (the U1 and U2, U11 and U12 snRNPs) are also the guideposts that bind near the splice junctions at the end of introns.  These help guide the splicing apparatus to the splice sites. There are also proteins that bind RNA and interact with the splicing apparatus to switch alternative splicing, such as SP proteins.
https://en.wikipedia.org/wiki/SnRNP
https://en.wikipedia.org/wiki/SR_protein
https://en.wikipedia.org/wiki/Exonic_splicing_enhancer     

Here's a recent review that might serve as an entry into the literature: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232567/