I am a newbie when it comes to play by post? How to begin? Which thread should I look into first? What is IC or OOC? How to roll die? How to create characters? What determine their stats/attributes?>
Is there any book I need to buy?

I am a newbie when it comes to play by post?

I don't think this is a question, but welcome to the hobby.

How to begin?

You could look through the Interest check section of the forums and join something that has yet to start, or sign up to an ongoing RP in one of their respective sections. Just post in the OOC if an RP looks interesting to you and you want to know if they are accepting new members.

What is IC or OOC?

IC is "In Character" which is where the story posts are posted. OOC is "Out of Character" Where the players/GMs discuss things. If you want to ask to join an RP, you would use the OOC tab.

How to roll die?

roleplayerguild.com/campaigns

How to create characters?

This is RP specific, but it involves filling out a character sheet. You'll need to look at the OOC/Character tab of an RP to see what's involved.

What determine their stats/attributes?

See above.

Is there any book I need to buy?

Usually everything will be in the OOC, but if you join something in the "tabletop" forum, you may be required to buy a book for the game system they are running.

Thanks.

How to know when you are required to roll a die or not in role-play? I read the dungeon and dragon's guide and discovered sometimes you need to roll dies.
Besides during combat, how do you know when you are required to roll a die or not?

The GM or Game Master will usually tell you.

It's basically any time you attempt something you could conceivably fail at.

You don't need to roll to walk or breathe or have a basic conversation.

You do need to roll to run up a sheet of ice that is about to crack off and sink into the icy depths, or to have the stamina to climb up that big ass cliff face so you can rescue that one dipshit Prince who went to go rescue the Princess but didn't have the upper body strength, or to convince the warlock that his Eldritch God is actually just an edgy teenager wearing some artfully modified chicken carcasses and speaking in French.

Another question, how do you know what to add to the attack? For instance, some posts I read add 2 or 3 or even 4 attack points to the dice roll. It is not consistent. Some players add 2, some 3, some 4. How do you know how to determine what points to add to your initial dice roll?

It varies by the rule set the RP uses.

Usually you have a Source Book, either the DM will tell you which one he is using or explain stuff. Most of this stuff can be found free online.

A lot of the +1 or +3 or whatever comes from picking skills for your character or finding gear. You might have an Exquisite Rapier that adds +1 to thrusting attacks and have the skills Fencer which also adds +1 to thrusting attacks.

So youd take the damage your weapon does (as found either in the Source Book the GM says you're using or by his explanation of the weapon) and add the +1 from it being exquisite and the +1 from your skill.

Usually you would track these things on a character sheet.

Ex of weapons
roll20.net/compendium/dnd5e/Weapons#c…

Ex of character sheet


Part of the GMs job is to give you the information you need to come up with a character. Tell you which Source Books you can pick from and they should be providing you with links where you can access enough of that stuff to faff yourself up a Character Sheet.