Hello and welcome to the forum!
Just like Firpi said, there is a topic in the "MH4U" section of this forum where someone asked for advice on the Longsword.
Here is a link to that topic:
As for weapons and armor you should go with.
For weapons, I'd most likely go with pretty much anything in low rank, as you'll be upgrading them throughout low rank and into high rank, so there aren't as many "better" ones.
Once you get to high rank though, the Crab Cutter LS (Longsword) is a decent one. It has good raw, adds a fair amount of defense (Which is handy.), and has nice sharpness, especially with the armor skill Sharpness +1 (Which that skill can be obtained in low rank, but a little later on in low rank.).
The Guardian Sword is also a decent LS with it's bit of white sharpness when you have the armor skill Sharpness +1. Although this one doesn't have the defense bonus, and has slightly less raw damage than the Crab Cutter.
Another LS that I would likely use is the Dios Katana. It can be obtained at the end of low rank, and has stats like an early high rank weapon.
That's just a bit of info on which Longswords I'd personally consider using.
As for armor for Longswords (Or skills, rather.), Sharpness +1 is a great skill for pretty much any Blademaster weapon for the most part.
Focus is also a great skill for LS, but it isn't an easy skill to obtain. The Evasion skill can be good, especially if you already evade allot.
The skills I mentioned are all good skills, but may not be very good since you just recently started playing MH4U. So you may end up going with more defensive skills instead, which is also fine.
However, if you want a low rank armor set with the skill Sharpness +1, here it is:
Helm: Ceanataur, Has one slot
Chest: Gore Magala
Arms: Ceanataur, Has one slot
Waist: Gore Magala, Has one slot
Greaves: Alloy (Or any greaves that have the skill "Torso Up")
Talisman: Anything you want.
That set gives you the skill Sharpness +1, has 3 slots on the armor (As mentioned to the right of the armor pieces that have the slots.), and lets you use any talisman you want. So you can gem in ("Gem in" or "gemming in" is a term people use when they refer to using decorations or "jewels" to acquire an extra armor skill on their armor.) another skill or two.