Also collect any weapons that have no modifiers and either no stars or stars in one category, they are really useful! They can be fused together to make weapons with stars only and eventually be used to isolate good modifiers from worthless ones.
Once you have a bunch of weapons, start combining them into weapons that have two modifiers you like. Don't worry much about the stars, because they can always be gained afterwards. If you fuse a weapon with all modifiers you need with a boss weapon (for example Cragalanche Cannon, not all weapons named after bosses are boss weapons, though), then by fusing the result with the same or a different boss weapon (depends on your weapon) you will get the same weapon you started out with, but with more stars.
An example would be my Fairy Orbitars: I started out with 5 stars in range, then I fused it with Phosphora Bow to get a 5.5 star Fireworks cannon with all modifiers present, then fused with a Phosphora bow again and got 5.5 star Fairy Orbitars.
When collecting weapons, keep in mind that not only big buff modifiers are good, but also negative modifiers are worth collecting. Why? They make passing on multiple modifiers easier and lower the value of your weapon so that when you use your weapon and get killed, your opponent wont get a lot of points. You also get way more points killing an opponent with a higher value weapon. In LvsD, the team gauge lowers by the amount of your weapon value when you're defeated. And coupled with a negative modifier passing Overall Defence +8 is much easier than without.
Also, if you have a second 3DS or a friend with a 3DS, then you can speed up your fusions by cloning weapons, because then you can not only keep your ingredients, but also combine your old results into new weapons with same or new modifiers.
To improve your skills you simply have to play the game more often, play against different opponents, play with different weapons and find what suits you best.
-> Collect a ton of weapons with good mods and stars in one category and play the game a lot.