WHIRLWIND EXPLANATION: Pokémon have a fixed, permanent score from 0 to 31 for each stat that affects how good that stat can ever get.

This calculator will figure out that score (called a "gene" or, more obtusely, an "IV") for you, so you can discard the unworthy. It's more accurate for higher-level Pokémon, so it's helpful to go into a level 100 wifi battle with someone and check your Pokémon's stats from there.

Stat calculator

Clefairy
Clefairy 		Base
stats		 Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Stats Effort Possible genes (IVs)
Level Level Level Level Level Level Level Level Level Level Level Level Level
HP 70
34–37
36–39
38–42
41–45
43–47
48–53
53–58
55–61
65–72
67–75
70–77
82–91
18
Attack 45
14–17
14–18
15–19
16–20
17–21
19–24
21–26
22–27
25–32
26–34
27–35
32–41
31
Defense 48
14–17
15–18
16–20
17–21
18–22
20–25
22–27
23–29
27–34
28–35
29–36
33–43
22
Special Attack 60
17–20
18–21
19–23
20–24
21–26
24–29
26–32
27–33
32–39
33–41
35–42
41–50
4
Special Defense 65
18–21
19–22
20–24
21–25
23–27
25–30
28–33
29–35
34–42
36–43
37–45
44–53
20–21
Speed 35
12–15
12–16
13–17
14–18
14–19
16–21
17–23
18–24
21–28
21–29
22–30
26–35
20

Hmm, I need more information to figure out your Pokémon's genes exactly. Try raising it to level 51 and entering its new stats.
Remember: if your Pokémon battles at all, you'll need to track the effort it gains. Consider saving your game and using Rare Candy instead.

And for your copy/pasting pleasure:

18 HP; 31/22 Physical; 4/20–21 Special; 20 Speed

or start over