Here is how I make my own movesets
1.Identify it's role
No matter what Pokemon you use, it always has a certain role that it's gonna fulfill. Usually the roles determine what to invest EV's in and moves into or the other way around.
Here are some popular roles that many use:
A Pokemon who uses physically offensive moves to do damage and bring down an opponent's team. Many carry Attack boosting moves, like Swords Dance or Dragon Dance.
A Pokemon who uses specially offensive moves to do damage and bring down an opponent's team. Many carry Special Attack boosting moves, like Nasty Plot or Calm Mind.
Pivot (Defensive / Offensive)
A pivot is a Pokemon that is generally only used for switching. Due to good defensive stats and a solid defensive typing, they can usually take little damage as they switch in, and the opposing switch they force allows the player to switch again safely to another Pokemon. An offensive pivot will force a switch by threatening KO on the opponent, thus obtaining momentum, whereas a defensive pivot will be difficult to break past, and thus will slow the opponent's momentum.
A Pokemon intended to take either physical or special attacks extremely effectively and consistently over the course of a battle.
A Pokemon who uses non-offensive moves which benefit the team.
A lead Pokemon, who usually pack low defenses, that tries to set up entry hazards quickly and prevent opponents from doing the same, usually by Taunt, before dying.
A sweeper who uses both physical and special offensive moves to do damage.
An offensively oriented Pokemon meant specifically for crushing walls rather than sweeping. This is usually done with extra powerful offensive stats and commonly use both physical and special moves in their moveset.
(Note: I got this infromation by looking on Smogon University's page about roles so if you want to see all of them then check out this)
2.Determine it's stat spread and movesets
Any Pokemon have a certain set of moves, whether it's a mve of their type or another. One move for each type the Pokemon you're using gives it STAB (Same Type Attack Bonus). While you spend moveslots for the type bonus, use the others either for coverage or for status moves.Coverage is to hit back against Pokemon that would normally KO you with one shot assuming the opponent move first. Status moves can be used to increase your stats, give the opponent a status effect, or to switch (usually for pivoting). For stats, you should invest some EV's into your stat depending on your moveset. If you want a fast and powerful physical attacker, put EV's into the Attack and Speed stat and the what's left of it into others.If you want a bulky Pokemon, invest in HP, Defense, and Special Defense.Natures can help increase a stat and lower another. For example, Adamant increases Attack by 10% and lowers Special Attack by 10%.
Abilities can be situational. But if it meets the conditions, it could give you a sweet reward. For example, Water Absorb makes the user regain HP when hit by a water-type move. Abilities can also be important in doubles.
Showdown! is a good way to test out a team you wanna use.Just ask a friend or family member to help you test out in battle. Showdown! includes many resources to set the exact things you want such as EV's, moves and Abilities of a Pokemon you wanna use.
I hope you find this helpful. I'll wish you luck in all future battles!