The other one was quite long and spammy, so I decided to restart it.
Before posting, make sure you check:
Walkthroughs for Diamond from GameFAQs
Walkthroughs for Pearl from GameFAQs
Or, this nice FAQ prepared by Volteon.
Q1 - How do I get National Dex?
A: You must see every Pokémon from Sinnoh Pokedex and beat Elite Four. Dialga/Palkia can be added in Pokedex reading a book in Celestic Town in Cynthia's house (Biggest house in city). Then all the other Pokémon can be seen battling with trainers in Sinnoh.
After you accomplish those, go to Rowan's Lab and he'll give you National Dex with PokéRadar.
~~A link about where to find all Pokémon for National déx~~
Q2 - How do I get to Route 224?
A: Get National Dex and go through Victory Road's last room. An entrance someone was blocking is now free and after a quest with Marley and her Arcanine, you'll get to the route. You may need HMs to go there.
Q3 - How do I get to Wi-Fi? Do I have to pay?
You need broadband internet and Nintendo's USB connector or a wireless router. Everything is well explained here.
Q4 - I battled with 12 Gastly holding Power Lens and it just got +2 Special Attack! What's wrong? Shouldn't it get +15?
A: No. The Pokémon doesn't get immediate EVs during a level up. EVs are gradually considered and they'll all be added when a Pokémon reaches level 100. Even if it's in Wi-Fi.
Q5 - How do I chain for shinies?
A very well explained guide even with videos is here
Remember the random encounter for shiny Pokémon is 1/8192. With PokeRadar, chaining to get a shiny is easier, but unfortunately you can't chain for overworld Pokémon, like Drifloon, Rotom, legendaries overall or Pokémon found in surf/fish areas.
Q6 - How do I soft reset?
Hold L+R+Start+Select. Soft resetting makes your battery live longer and works faster when compared with just Turning off/on.
Q7 - How can I get Feebas and how can I evolve it?
A: Feebas is probably the rarest Pokémon found in wild. They are found in Mt.Coronet. Fishing with Old Rod will give you higher chances to find it faster because all this Rod gets is Magikarp and Feebas.
To evolve Feebas is reccomended you save before feeding Feebas Poffins. Then give it Poffins made by Ganlon or Pamtre berries. Ganlon berries can be found in 3D games (Colosseum, XD, PBR) and Pamtre can be found in Amity Square random encounter.
There's this video in YouTube that will help you.
Q8 - Do I need Wi-Fi to go to Sinnoh's Underground?
A: No. DS will simply start it's wireless function, you won't need Wi-Fi or someone next to you to go Underground.
You can go there by yourself to dig for fossils and create your secret base.
Q9 - I tried to release an Azumarril but it returned!? Is my game glitched?
A: It's not glitched. When a Pokémon knows an HM that no other Pokémon has in your box and you release it, the Pokémon will return. Just teach the HM it knows to another Pokémon and it should be fine.
Q10 - I deposited a Pokémon in GTS and after a few months I went check GTS and it disappeared!?
A: It is normal. It is explained in Pokémon Diamond/Pearl's guide that comes with your game that a Pokémon can disapear from GTS.
Q11 - Where can I find Move Relearner, Nickname rater and Move deleter?
A: Relearner is in Pastoria City, in a house near the lake.
Nickname rater is an oldman in Eterna city in building next to the market.
Move deleter is in Canalave Town in a house below Pokémon market.
Q12 - Where can I find <insert TM here>?
A: Check this. There's a complete list there.
Q13 - I heared Pokémon that requires trading can evolve without trading with someone, is that true?
A: Yes. more info in this thread.
Q14 - How can I do the cloning glitch in DP's GTS? Is it safe?
Check this thread for it. And yes, you must not worry about your game file and must not pay attention to those who said file got corrupted. it's 100% safe.
Please don't post repeatedly, you'll get help eventually. >_>;
Also, please don't post "thank you" posts. These are spammy. Edit your original post with the thanks. ^^;
Also, don't post questions that have been answered by the FAQ. You will be infracted