The way characters gain levels is set up pretty much like every other Final Fantasy game. Characters are awarded Experience points after they defeat enemies. After gaining a set number of these points a character will gain a level. When this happens the characters stats will change. They will get more Hit Points, their strength and defence will get stronger, etc.

The harder the enemy party, the more experience you will gain. You will have to gain a lot of levels to get through FF 1. You can't just play the game through without taking some time to do levelling up. The enemies get hard really fast. Take time at the beginning of the game to level your party up. It's the easiest time to do it because the enemies are weak.

Level 50 is the highest attainable level. You can't go up to 99 like in later final fantasy games. Your characters experience will max out at 999999 and will stop there.

Level 1 - 25
Level
Experience
1 0
2 40
3 196
4 547
5 1171
6 2146
7 3550
8 5461
9 7957
10 11116
11 15016
12 19735
13 25351
14 31942
15 35986
16 48361
17 58345
18 69617
19 82253
20 96332
21 111932
22 129131
23 148008
24 168639
25 191103
Level 26 - 50
Level
Experience
26 215479
27 241843
28 270275
29 300851
30 333651
31 366450
32 399250
33 432049
34 464849
35 497648
36 530448
37 563247
38 596047
39 608846
40 661646
41 694445
42 727245
43 760044
44 792844
45 825643
46 858443
47 891242
48 924042
49 956841
50 989641