How to choose step up transformers?

I just get into vinyl.  I bought a Sony PS2250 turntable and I have a Luxkit a3300 preamp.  The preamp has a MM phono stage but not MC.  I would like to get a MC cartridge so I am looking to buy a Step up transformer.  I don’t know what MC cartridge I am going to get yet.  Do I need to match up the cartridge and the SUT?  If yes, what do I need to match?  Likely I will buy something not too expensive such as the Denon DL301.  
“Do I need to match up the cartridge and the SUT?”

I would say Yes! The two important numbers in MC cartridge to be aware before you go SUT hunting are the internal impedance and the output voltage. I would suggest reading up on Dave Slagle’s primer on SUT selection,

The two manufacturers I know off that have excelled themselves in providing custom SUT tailored for your cart are,

Lot of folks also like Auditorium A23 with Denon carts.
Hope this helps!
i agree with @lalitk .........if you are a DIY kinda person who gets techie stuff.

if you really want to do it right and are not techie, contact Dave Slagle directly and ask him to recommend which of his SUT’s is right for your cartridge and phono stage.

in my case, he sent me two to try, then two more, and i’m really happy with the results. i now own 2 of Dave’s SUT’s.

i wish i was smart enough to figure it out on my own, but i could not quite get a handle on it myself even after reading a bunch of stuff. if that is you too, don’t let that push you away from SUT heaven.
Thanks for the reply.  It seems I need to first pick the cartridge then buy the SUT.  But if I change the cartridge, I may need to buy another SUT?

The good news is that SUT matching is not that critical! So if you change your mc cartridge you will still have very good-to- excellent sound quality - just with a different sonic "flavor". That was what the late - and missed! - Art Dudley concluded after years of listening. He mentioned this in one of his many columns!
I own five SUTs, including a Mike Sanders Quicksilver - and numerous mc cartridges. Why? For the different sonic "flavors" with different combinations of cartridges and SUTs. 
And for something "different" I also have a Marcof PPA-1 battery-powered SS head amp. This too sounds excellent! 
While a generic SUT covers a broad range of MC carts (2-40 ohms), a purpose built SUT from EMIA and Bob Services will definitely sound better with the cart it was originally spec for. That’s how one ends up with multiple SUT’s. When it comes to Analog, you want to extract every last ounce of goodness from that cart and vinyl.

Glad to see roberjerman loves his vinyl cause he pretty much trashes everything else here :-)

Check out this thread as well,
@lalitk : Yes, I am critical of most of the claims made for today's Hi End gear! But I am content to live with and use some of the best tube and SS gear of the pre- '90 eras! Three roomfulls, in fact! Plus many TTs and boxes of LPs. And not to forget the three RTR's and boxes of RTR tapes!
I have been using Grado MI's and various MC cartridges since 1976 - so I certainly have experience to draw upon!
Not sure if using a low-end MC is worth all the connections and lossy wiring over a good MM. Especially in your setup. Compare an Audio Technica VM760 with the MC configuration. 
GTE, it is basically a matter of impedance. If you buy a transformer matched to low impedance cartridges you should stay with low impedance cartridges. Same for higher impedance cartridges. I think you will find this very helpful
If you know someone who is experienced with a soldering iron you get the appropriate transformer from Sowter, a british company who makes excellent products, and you have him mount it inside your Luxkit. They are quite small. Then you do not have to worry about interconnects. The transformers are around $100. Even if you do not want to do it that way visit the site as it is very instructive in lay terms.
I have put their transformers in several preamps including my own Conrad Johnson with great success.

I read the links mentioned above and honestly, I can’t figure it out.  I decided to buy a Denon DL-H5LC cart.  
It has a low output of 0.3mV, so, I need a high gain SUT, like 1:20+ ratio.  What I don’t understand is the impedance part.  It has load impedance of 100 ohm.  Unload impedance of 18 ohm.  What is the significance of these two numbers?  It seems the impedance is related to the gain.  Many SUT I checked has just a hi/Lo gain setting.  Some have a ohm setting such as 10 ohm:31dB (1:36) and 30 ohm:26dB (1:21).
 What about that secondary resistance?

So, besides choosing one with a higher gain, what else I should consider?

It is only the cartridges impedance you care about which is 18 ohms. This is a higher impedance cartridge. Get a 1:20 or above transformer for higher impedance cartridges. The load impedance concerns the SUT or phono stage. Now, go back to Souter and see if you can pick out the right transformer!
Don’t worry about the secondary. That is determined by how much gain you want.

The 18Ω number is the cartridge internal impedance and essentially represents the DC resistance of the coils.  This in effect is the source impedance driving the SUT and dictates the frequency behavior at both extremes.

The 100Ω load impedance is a number that changes meaning from manufacturer to manufacturer.  Some consider this value an absolute maximum value load that can be applied and still give results that are considered acceptable. (a maximum load is a small resistive value) Other manufacturers consider this value to be the value required for optimal performance and is typically given as a range. (ie 470Ω-2000Ω).  In either case this is a very soft number based on some unknown parameters and typically a final value should be determined through experimentation.  In the case of the Denon, the cartridge database states >100Ω suggesting it represents the maximum load and anything between 100Ω and 47kΩ would be acceptable.  In reality, if you try 50Ω and like the sound there is nothing to worry about.  

The nature of a SUT transforms the (typical) 47KΩ input of the phono by the turns ratio squared and presents that load to the cartridge so a 1:10 turns ratio has a 1:100 impedance ratio so that 47kΩ appears to the cartridge as 470Ω.  Moving to a 1:20 gets you a 1:400 impedance ratio and 117.5Ω which still is within your manufacturers acceptable range.  Going to a 1:40 nets a 1:1600 impedance ratio or around a 30Ω load which is well below the 100Ω number but I know of a few people who use a DL-103 with its  >100Ω recommended load with a SUT that can be switched between 1:18 and 1:36 and prefer the 1:36 setting even though the reflected load is actually less than the cartridge internal impedance (36Ω load on a 40Ω cart).  This situation makes the number counter scream since 36Ω is 1/3 of the manufacturers  maximum load. Then consider that a load that is equal to the internal impedance also results in a 6dB reduction of gain.  This works out so the volume level in this situation is only about 1.5dB higher for a 1:36 vs. a 1:18.  The truth of the matter is since it is easy to break the rules and flick the switch, 90% of the people will simply leave it on the setting that sounds best and if that happens to be the 1:36.... then so be it.