How to understand and analyse RNA-seq data (for a beginner)?
SRA files are raw sequencing data that have been converted to the NCBI storage format. These can be converted to fastq files, e.g. with fastq-dump from the SRAtoolkit. 

Beyond that I suggest you start with the Hitchhiker's GUide to RNA-seq and the DESeq2 workflow to get a background. Be sure to extensively browse the web for vignettes, guides and tutorials. YOu will find helpful resources to get started.