Bioconductor has a great tutorial that walks through the whole process. You can find it here. Also refer to the DESeq2 and edgeR vignettes.

Do you have experience with linux comand line? If you don't that may be where you may want to start. Here is a simple tutorial for that: http://korflab.ucdavis.edu/unix_and_perl/current.html#part1

If you are not able to use the command line then using online Galaxy may be an option: https://galaxyproject.github.io/training-material/topics/transcriptomics/tutorials/ref-based/tutorial.html

If you are willing/able to spend some money then Biostar handbook may be an option to look at.