Before buying check out different brands to see which are the easiest to read for you. I find the yellow rulers very difficult to read, as well as the clear with black writing. I find Kay Englands's Nifty Notions rulers in teal and pink easiest to read. Look at the True Cut rulers as well. Their custom rotary cutter rides on a raised edge on the ruler. The colors are similar to Kay's and the edge eliminates inaccuracy from tipping the cutter when cutting. They also sell little round stickers to put on which reduces ruler slippage. I have found only one shop that carried them. I wanted to try it before purchasing and loved them and highly recommend them for accuracy.
I suggest starting with the 6x24 size. If you fold your fabric in half, selvage to fold, it will cut your strips accurately. Take some time to look around. It is good to have your basics all the same brand. Rulers are an investment and will last for a long time. Choose what works best for you, don't just settle for the cheapest