Hi Michael Antony,
As mentioned by Ramit and Ludwig, there are plenty of discussion you will find in this Forum which you can refer on this topic.
So, from my side nothing more to add. You can either use excel add-ins if your data volume is comparatively low. If you are dealing with high data volume, then it's better to use Data Management.
You can use General journals data entity to upload TB (this is GL balances), Customer, Vendor and Bank balances.
For PO, you can use Purchase order headers V2 and then Purchase order lines V2 data entities. Similarly, you will find for Sales order, Production order and also for Inventory data entities in Data Management.
To upload Fixed assets balances, you can use Fixed asset journal data entity.
A vital point that you need to keep in mind that, while uploading GL balances, you have to link financial dimensions with the main account in AccountDisplayValue column in the upload template.
Here below in my example, 400 and 430 are the BU and DEP financial dimensions.

But in case of subledgers like Customer, Vendor, Bank and Fixed assets, you have to put the same dimensions in DefaultDimensionDisplayValue column in the upload template like this '-400-430-
Now this " - " depends on how many financial dimensions you have validated and linked with Main account in the 'Account structure' and in the "Financial dimension configuration for integrating applications' forms. As in my case, the main account always comes first. So it starts with a " '- " and then dimension values as per the sequence are set in above forms.

You can generate upload templates for each data entity directly from Data Management after clicking on "Import". In case if you are not aware of that, then you can check this with your team mates.
Kindly check and update on the outcome.
Regards,
Sourav