On each piece of gear, there are caps for each stat. Stats are divided into main stats and sub-stats.
Main stats: Vitality (VIT), Strength (STR), Dexterity (DEX), Mind (MND), Intellegence (INT)
Sub-stats: Critical Hit Rate (Crit), Direct Hit Rate (D.Hit), Determination (Det), Skill Speed (Sk.Sp), Spell Speed (Sp.Sp), Piety, Tenacity
—HQ white gear (high quality gear with a white name) or HQ green gear (high quality gear with a green name)—HQ gear has a little icon next to it) and the majority of other gear (e.g., dungeon gear) on your left side, the main stat (VIT/STR/DEX/MND/INT) is always capped and will not allow you to meld main stats onto them.
For right side gear (accessories), it is possible to meld VIT into HQ pieces or endgame pieces for additional health, and tanks can meld STR into their accessories. However, melding main stats into your accessories is generally not needed unless you are doing high-end content that requires it. For casual content, you do not need to meld VIT into accessories, nor is is mandatory for tanks to meld STR. So I wouldn’t concern yourself too much with this right now. ^^
—NQ (normal quality) gear also has a cap, but it allows you to meld main stat materia to make up for the fact that it isn’t HQ; NQ gear is not capped on main stats pre-melding.
Generally speaking, when you reach level cap, you want to try and make use of end-game gear rather than NQ gear. HQ gear will always be better than NQ when leveling, and since you cannot meld main stats into HQ gear due to caps, that is why you generally don’t see people melding the main stats.
For other sub-stats (Critical Hit/Direct Hit/Determination, Skill Speed/Spell Speed, Piety/Tenacity), there are caps on the gear, and they can be determined a number of different ways. The easiest way, in my opinion, is to look at a piece of gear, and make note of the highest substat listed. This will tell you the maximum that you can meld for the other substats.
So, for example, we’ll look at a level 40 body piece (so, something close to your level):
Woolen Shirt (i40 body for Disciples of War)
NQ stats are listed first, with HQ stats in parentheses
DEX: 8 (9)
STR: 8 (9)
VIT: 8 (10)
Direct Hit Rate: 11 (14)
The highest substat here is Direct Hit, which caps at 14. This means that you can meld up to +3 additional Direct Hit (for NQ only; you will be unable to meld Direct Hit if the shirt is HQ), and up to +14 of any of the other substats. Generally, for DoW, you would meld either Critical Hit or Determination or Skill Speed. Spell Speed is only for DoM, Piety can only be used on healers, and Tenacity can only be used on tanks.
I will include an endgame example of melding under the spoiler, since endgame gear is a bit different compared to leveling/baby gear. I’ll even choose a body piece for BRD, since I see that’s the job you have listed on your profile. ^^
True Linen Jacket of Aiming
Again, NQ stats are listed first, with HQ in parentheses
DEX: 264 (293)
VIT: 282 (314)
Critical Hit: 239 (265)
Determination: 167 (186)
On NQ pieces, you can meld DEX or VIT onto the piece, up to the HQ gear stat listed within the parentheses. You cannot meld other main stats such as STR, MND, or INT, and there would be no need since BRD’s main stat is DEX. (All gear has VIT on it to give you more HP.)
The maximum amount of any substat (Critical Hit, Direct Hit, Determination, or Skill Speed) that you can have on this piece is 265 points—you go off of the HQ stat for the Critical Hit substat, and that is your cap.
For Critcal Hit, if your gear piece is NQ, you can meld 26 more points into it. If your gear is HQ, you cannot meld anymore Critical Hit.
For Determination, you can meld either 98 points for the NQ version, or 79 points for the HQ version.
For Direct Hit or Skill Speed, you can meld up to 265 points of each.
Another way to determine the kind of melds you can do is to look at websites like Ariyala, which will allow you to simulate gearsets and meld them to see what kind of stats they will give you, or Garland Tools, which will list all the items in FFXIV’s database, and give you values for each stat currently on the gear, and the stats caps for them. It also tells you the difference between NQ stats and HQ stats. A third way is, when you go to meld items, you can look at the bottom of the window that appears listing all the gear you have available to meld, and each stat and their caps will show up at the bottom.
For the NQ Woolen Shirt, it would look something like:
DEX: 8/9. STR: 8/9. VIT: 8/10. Direct Hit: 11/14. Critical Hit: 0/14. Determination: 0/14. Skill Speed: 0/14.
Hopefully this makes sense. ^^; I tried to make the explanations as simple as possible, and not use too much jargon since you said you were new, and I didn’t want to start throwing around terms and abbreviations you may not be familiar with. I would recommend, until you get more comfortable reading the gear stats and automatically determining which stats are capped, which you can easily meld, and how much of each, using Ariyala or Garland Tools to help out.
But really, melding is not that important until you get to level cap, because you go through leveling gear so quickly; and only if you plan on doing end-game content like Extreme trials, Savage raids, or Ultimate. In the majority of other content, melding is not that important. So the information is useful to have, but I would not fret too much over it until you reach level 70. :3