Fish that's been previously frozen works perfectly well. As with most meats, it's actually easier to cut when it's still slightly frozen.
As for how to make it, it's not particularly difficult. I'm assuming you're talking about making the maki (rolled sushi).
You'll need the nori (the seaweed sheets) which can be found in most grocery stores in the US now days. For rice, make sure to use short grained rice. Often times, you'll be able to find it in the asian section with the nori.
Cook the rice according to directions and season it with rice vinegar. Spread it on a sheet of nori, leaving about 1cm (~1/2 inch) at one end. Make sure to get the rice right up to all the other edges. On the end opposite the one you left some space at, spread your toppings. If they have any sort of preferential direction (like cucumber strips), make sure they're parallel to that edge.
Then, starting from that edge, just roll the sushi up. For normal sushi with the nori on the outside, you don't really need a rolling mat, but they help. Make sure to roll it as tightly as you can. If the edge doesn't tuck in as you try to roll, take a long knife, and with the side opposite the blade, push it under and slide the knife out once it's tucked.
If you're using the mat and haven't used one before, make sure you roll it back on itself so it doesn't get rolled in the sushi! (I was dumb enough not to get this my first time...)
Inside out sushi (sometimes called California rolls, although that can also refer to the fillings) are trickier. For those, you must have a rolling mat. Cover it with plastic wrap and then lay out the rice just as you would have on the nori, but with no nori present. Once the rice is laid out, lay a half of a nori sheet on the top edge. (Full ones will be too long and stick out oddly when you're finished.)
Again, lay toppings long one edge and roll.
Cutting the sushi can also be very difficult. Make sure your knife is exceptionally sharp. I have a special sushi knife that's only used for sushi and its ingredients. When cutting, don't push down. Use saw like motions with a very gentile pressure. The weight of the knife is almost all that's necessary. If the rice sticks to the knife, clean it and run some water over the knife.
That's about all there is to know for maki!
Here's a list of common fillings:
Seafood
Tuna
Salmon
Yellowtail
Squid
Eel
Octopus
Shrimp
Clam
Roe
Crab
Vegetables
Daikon radish
Avocado
Cucumber
Asparagus
Yam
Gourds
Snow Peas
Other
Tofu
Eggs
Cream Cheese
Mayonnaise
Sesame Seeds
Furikake
it's very useful, thanks for sharing
) Sushi Magic! 
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