Nos

I Have A 99' Sentra And Want To Put Nos In It, Do I Have To Mod The Eng Any Or Can I Just Install It?

It's nitrous, not nos. NOS stands for Nitrous Oxide Systems, it is a company, not a product. Every part of your question depends on what engine you have, what size shot you plan on doing, and if you want to go dry, wet, or direct port.

i hear that on a qr25de an u can do upto 50 shot of nitrous, is that wet dry or direct port, or does it even matter which i pick?

I think 50 would be alright on the qr25, and with that engine you have to do a direct port I believe (no fuel return line)

It's nitrous, not nos. NOS stands for Nitrous Oxide Systems, it is a company, not a product. Every part of your question depends on what engine you have, what size shot you plan on doing, and if you want to go dry, wet, or direct port.

1.6L ENGINE, I WANT T0 DO A WET. I HEARD THAT DRY NITROUS WILL BLOW YOUR ENG WITHIN A YEAR OF USING IT. SOMEONE ONCE TOLD ME THAT WHATEVER HP YOU HAVE SPLIT THAT AND THATS WHAT SHOT NITROUS YOU SHOULD USE. 100 HP / 50 SHOT

Yeah I would do a max of 50 wet, or maybe even a 75 shot direct port. Whoever told you that 50% thing needs to learn more before they go off spouting their mouth, lol. You can't just do that for every car and have it work, there was a guy with an 01 sentra se like me running a 125 direct port on full stock internals, this is on a car that comes stock with 145hp. With full bolt-ons he dyno'd 290whp and over 300wtq on the juice. And according to that formula you would feed a qr25 almost a 90 shot, which would almost certainly snap your rods on the first spray.

I Have A 99' Sentra And Want To Put Nos In It, Do I Have To Mod The Eng Any Or Can I Just Install It?

Ok, guys let me clear up a few misconceptions on Nitrous. Wet or dry does NOT matter. I your fuel pump goes out you'll be buying a new engine. the only real difference is a decision of weather you would rather cool your intake or provide a better catalyst to the flame, about the same really.
As for what is safe. Well define safe. Just depends on how long you want your engine to last. But using Nitrous on cast pistons causes puts divits in your piston, causing heat build up and can eventually blow through it. So I would change your pistons If you are concerned about your engine, while your at it get a set of large gap moly rings. better for Nitrous.
Even after your change that, as a rule of thumb cars are made to withstand 150% of their stock potential. eg: 130 hp car, 65 shot max, with some serious engine work. Also, NOS kits come with a sheet that will tell you safe HP levels.