Stick with one brand. I would get a 6 x 12 or larger to start. I am a bit anal and buy the different squares depending on the project. I can mess up anything so if I need a square square I buy the size I need. I also LOVE LOVE LOVE the slotted ruler I have that is slotted at 1/2" increments.

Creative Grids for the thin lines and no-slip feature and June Taylor Shape Cut. I have 25 kajillion rulers and I find myself reaching for the June Taylor every time I cut strips for a project. I can have 6-8 strips cut in less than 30 seconds and all are perfectly accurate. Take a look on You Tube for the June Taylor videos. That's what convinced me.
However, I like having the ruler I need for the job I'm doing. Sometimes, I need a very small, maybe 3" square for working while at the machine. Sometimes, I need a 24" square for block squaring. I even bought one that unfolds into a large right angle and have used it several times to square up quilt corners. But June Taylor is the most used.

June Tailor Shape Cut Plus will give you more bang for your buck than the Shape Cut. Once you start using these slotted rulers you won't use anything else.

The Martelli mats are just the best. Thicker than the Olfa and Omnigrid and you don't have to store them flat. And they can take the heat and won't get warped. Pricier than the others but I feel like I've gotten my money's worth.

Make sure you can easily see the markings on the ruler. I have one with black markings and I hardly ever use it because it's hard to see the marks on medium - dark fabric. I much prefer to use an Omnigrid that's the same size, because I can see the marks much easier. Some rulers are not necessary to make a pattern, others make it easier &/or faster. Probably every person on this Board have rulers they've bought and never used. Spend your money wisely.

I have every size square made but dont use them much. Have the shape cut for strips and dont use it.
Some specialty rulers like the x box and never used them.
I use both my 6 in by 12 and 24 the all the time. Both by the same manufacturer.
The specialty ruler I do use alot is Quilt In A Days triangle square up ruler. Love it. One cut for a perfect half sq triangle.
I do use a large 16 in square ruler for squaring up the corners of the quilt then use my 6 by 24 for the sides.

I only use Omnigrid/Omnigrip rulers as all of the other rulers confuse me, I'm number dyslexic.
The reason to use one brand of ruler is some rulers are off slightly and it'll really mess up your quilt if you switch brands.
I used to teach beginning quilting and the best rulers to start with are: 6"x24",6.5" square and 6"x12". Also the Olfa mats, I don't use the lines on the mat as I'm not using the Olfa rulers.