Start with the best tools you can afford -- if you have an inexpensive rotary cutter or a bad mat you will hate it and you will probably not be able to cut accurately
Take good care of your machine -- clean it and oil it (if it needs it) frequently and it will like you.
To get an accurate 1/4" foot, use a lined note card (the lines are 1/4" apart) and put blue painters tape on the bed to mark the spot. I did this for years before I had the 1/4" foot and it really helps.
When cutting things out, decide how are you goin to position the fabric with respect to the ruler line to get your measurement -- on a side or maybe in the middle. If you aren't consistent, your blocks won't be the same size.
Start with fabric from a thrift store so you don't feel bad if you make a mistake and have to toss the block. This is good practice material and a lot of folks here on the board use what the get at the thrift store or garage sales all the time since fabric is soooo expensive.