This is a simple guide intended to inform players what happens as their characters grows up in level. The guide focuses on fundamental game features you must deal with. I'm not covering details of actual recommended quests/actions for certain class level.
While making some brief notes on this topic just for myself, I was rather surprised to find the level up process is not as simple as I thought. That's the reason I decided to expand the notes into some guide to share with other players
My appreciation goes to Japanese PSO2 Wiki (https://pso2.swiki.jp/). I got the XP data and many details of client orders from there.
Please note that a good deal of info listed in this article is based on what they are now in JP server. Some points can change in NA server. The amounts of XPs listed are total XPs you need to earn to get to that level starting from level 1.
English is my second language, so please forgive me if I use some bad English.
[Level 3] (XPs needed: 480) You can now have your own Mag. Meet Cofy. Maybe you want to check out some guide for Mags before you actually start feeding items to it. Mag breeding is a common pitfall for new players of this game.
[Clear "Forest Exploration"] This is not a level milestone, but it's quite an important one worth to mention in this list. The enemies of forest area are about level 5-7 (need confirmation). If your level is higher than this, you won't have much problem clearing the quest I suppose.
After clearing Forest Exploration, meet Cofy and she'll give you permission to increase the number of weapons. She also gives you permission for hard difficulty quests. But most of the hard difficulty quests also requires you to be level 20 or higher, so you probably won't be able to select hard difficulty for at this point.
[Level 20] (XPs needed: 145,620) Quite a lot of things happen here. You can now set sub class. Meet Cofy to accept the client order to unlock this feature. Your sub class can gain some XPs along with your main class, so it's recommended to visit class counter and set your sub class ASAP.
Then, if this is your first class to hit level 20, take a client order from Cofy to raise your level cap from 30 to 40. A client order to increase the number of sub palets is also available at this timing. Finally, you can now take hard difficulty quests.
[Level 30] (XPs needed: 614,120) After reaching level 30, the class mentor of that class will have a special client order with a reward of 5 extra skill points for that class. If you are planning to play that class frequently, taking this client order is highly recommended. If you are just raising the class level for various bonuses and not going to play the class, then perhaps you can ignore this.
[Level 35] (XPs needed: 983,420) If this is your first class to hit level 35, take a client order from Cofy to raise your level cap from 40 to 75 (confirmation needed). This is a very important step. If you don't complete this client order, you'll be going to waste your XPs after hitting level 40.
Also you can get the permission for very hard difficulty quests from Cofy. Again, you have to be level 40 or higher to acutally select very hard difficulty in most of the quests.
[Level 40] (XPs needed: 1,394,720) You are now eligible to get intermediate rank daily/weekly ARKS Missions. Note that missions you currently have won't change. Daily missions of new rank will appear from the next day, new weekly missions will appear after the next maintenance. Also, very hard difficulty is now available.
[Level 45] (XPs needed: 2,250,000) Class mentor will have another client order for 5 extra skill points. Again, if you're going to play the class seriously, it's a highly recommended order.
[Level 50] (XPs needed: 4,517,500) Super hard difficulty is now unlocked. You will get a message from Cofy to tell you about super hard difficulty. After receiving this message, you will also receive advanced rank daily/weekly ARKS missions starting from the next ones. The advanced rank ARKS missions offer quite a good reward, like 400,000XP for completing all daily missions. So having at least one class of level 50 will help you greatly.
[Level 65] (XPs needed: 21,540,000) This is not a very important milestone. You can now receive a series of 4 client orders from class mentors which gives you a total of 4 skil points. But these client orders requires special item "Class EX-Cube" to complete. In NA server launch, the only way to earn Class Ex-Cube is to get level 75 and level up furthermore (confirmation needed), so you can ignore these client orders for now.
[Level 75] (XPs needed: 39,357,500) This is the highest level you can achieve in NA launch. Congratulations!!!
If you reached this level, visit title keeper Lachesis. She will have some nice title rewards for achieving level 75. Among these rewards, there's a title of class boost, which gives you some bonus on your stat. This class boost is applied for other classes than the class you've achieved 75, and it also applies to other characters you have. Each class have its own class boost and they all add up, so the more classes you have at 75, the more better stat modifier you'll get.
Also at level 75, you can earn Class EX-Cubes by filling the XP to max again. On some old info I could find on JP server says 1,967,000 XPs are needed to get a Class EX-Cube at level 75, but honestly, I have no idea how this goes on NA server.
This ends the list. I hope the guide will be of some help.
For reference, here is the list of class mentors. Hunter & Fighter - Oza; Ranger & Gunner - Risa; Force & Techter - Marlu; Braver - Azanami; Bouncer - Saga/Katori; Summoner - Pietro. All of these NPCs can be found in the first floor of Gate Area.