How to expand fast

[shower_room1944]

I attack and attack but people expand faster than me and idk how

[Marquoz]

1) Idea groups. If you want to expand fast, your first three groups should be Diplo, Admin, and Influence. Diplo lowers the warscore cost of provinces so you can take more land in each war. It also gives you more and stronger diplomats to forge claims and improve relations, which will stop coalitions. Influence is for vassal annexation--you want to grow both by conquest and by annexing vassals so that you use two types of mana instead of one. And Admin is to make both conquest and vassal annexation cheaper--Admin lowers coring cost by 25% and also combines with Influence to enable you to enact a policy that lowers vassal annexation cost by a total of 45%.

2) Reconquest wars. These are a HUGELY important tool. Many nations don't exist at game start, but you can conquer one province of what can be their land, release them as a vassal, and then fight reconquest wars for the rest. Those wars generate very little AE and require far less warscore per province than normal. Examples of such nations are Gascony, Syria, Bulgaria, and Kazahk, but there are many many more. Other nations (like Byzantium and the Timurids) will cease to exist when they get eaten by their neighbors. But again, you can grab one province, recreate them, and fight reconquest wars.

3) Experience. The people who expand faster than you are experts at the game with thousands and thousands of hours of experience. You have a tiny fraction of that. You'll get better with time.

[NewVocat]

On the topic of idea groups: it is quite possible that you will face some stability issues or rebels in your country especially if you expand fast and the 2 idea groups that help the most at that part are usually either humanist or religious.
Humanist is great for unrest reduction, tolerance of heathens and heretics and lower seperatism for even less unrest in newly conquered provinces.
Religious ideas are a bit tougher to play in the lategame if you conquer many provinces at once and need the extra micro of converting provinces but will help with higher tolerance of the true faith to keep rebels from spawning in provinces of the right religion and converting all your provinces to the right religion. Religious ideas are also quite useful due to the prestige gain on religious conversions if you are playing with vassals or force convert those to spread the right religion.

If you are somewhat familiar with horde mechanics or want to learn horde i would recommend playing Kazan the neighbour of Great Horde, Muscovy and Uzbekistan and go for admin, diplo, humanist ideas, it is quite easy to expand quite fast with that setup and to have a very stable nation without converting.
For religious playthoughs there are many options, coptic Ethiopia is quite interesting as they get additional missionaries in their national ideas and from the holy sites nearby and coring cost reduction from the religion and their national ideas, the start is a bit tough as there are many neighbours of the same (wrong) religion but it is possible to overcome and convert them.