@nieshxa
Some plants need the required number of seeds to unlock, most powermints need 100 mints to unlock but now seeds can be obtained but you still need to purchase the powermint with mints.
Some plants can be purchased with gems and some with cash or Google play credit (android only).
Currently the only powermint for sale is Bombardmint.
Below are screenshots of what gem plants are available and Bombardmint.
If you Google pvz2 plantname then you can visit the fandom site that has a description of the plant plus a table showing the costs of levelling up.
It costs 4,150 mints/seeds to max out a powermint and the best levels are 4, 7 and 10 because they knock time off the refresh rate.
I have set up a hints and tips thread. https://answers.ea.com/t5/Plants-vs-Zombies-2/HINTS-AND-TIPS/m-p/6874001#M3279 but some of the info is out of date as EA/popcap makes changes without notice.
There are some very useful forums, the level 1 mini forum and mid level mini forums would be quite helpful to you.