1. Knight are mostly STR based. STR increases your DEF and the amount of HP you have. You can add DEX (Attack Power) or CON (Accuracy) if you'd like. But that's for every man's own.
2. Yes, you will have to talk to Mildun (can be found in Junon Polis) and he will give you the quest.
3. Investing in a Union is always a wise thing to do. Participating in Union Wars grants you points if your side wins. With these points you can buy special Union Gear, which is mostly stronger then the gear that is sold in NPC shops or dropped by monsters.
4. Leveling up is done by looking for a good party to join. At your level I would investigate Kenji Beach for a fun party. Claninvites is done by asking a clanowner or wait for you to be invited. Don't ask to much though, if the clanowner sais no, don't go begging. A wise thing to do is to wait for you to be at a much higher level before joining a clan right now.
5. You will need to farm by killing monsters for gear with extra stats. For HP gear you can go for reinforced armor. Reinforced armor is obtained by asking a Artisan to craft you some if you've got the mats needed for it. Gear that adds STR for soldiers is called Legendary Gear (dropped by monsters)
6. No, there are 4 planets now (Junon, Luna, Eldeon and Orlo), The last 3 planets are still being developed.
7. Never beg for money or items. It makes you less popular and it won't work.
7.2 It's easy to refine your gear to (5). Virtually costs nothing and is easy to get to,
7.3 Enjoy the game