There are several ways to express the idea of something being under your authority (as opposed to you just owning them). The most basic is the word potestatis. So, for example,
...in potestatem Romanorum transiit (it passed under the authority of the Romans)
You can also use the word auctoritas:
...ad unius se auctoritatem contulerunt (they put themselves under the authority of one person)
...ad P. Lentuli auctoritatem a senatu profectam Roma contenderat... (...he left Rome under the authority of P. Lentulus which had been granted by the Senate)
The word dicio can be used to indicate authority:
dicionem populi Romani sese tradebant (They put themselves under the power of the Roman people)
The way that you say "all my stuff" in the sense of wealth is omnis mea ops or mea res omnis. The way that you refer to petty possessions and everyday things is the neuter noun bonum (a good). For example,
ego huc bona mea degessi (I brought my stuff here)
As Koppehel says above, the Romans did not typically think of possessions being under the "power" or authority of somebody. In Latin, power is wielded over people, not inanimate objects. However, there is a special word in Latin which is used to describe control over property and that word is the 4th declension manus (the HAND, kind of like THE CLAW in Toy Story). So, for example, we have this statement from Cicero:
Cum mulier viro in manum convenit, omnia quae mulieris fuerunt viri fiunt dotis nomine. (When a woman goes under the hand of a man, then all of the woman's belongings become the man's by way of a dowry).
Now, before attacking the main thrust of your motto, there is another important idiom to be aware of. That is you can use meus to indicate something belonging to you, but when speaking emphatically, the word noster is used. In other words, noster usually means "ours", but in an emphatic or emphasized context, it means "mine". So, now that we know this, we can put together a Roman motto to get your meaning:
Omnis res nostra mihi in manu [est]. (Everything of mine is under my power.)
Note that you can omit the est as understood to increase the poetic effect of the motto.
Finally, I should probably explain why we have mihi instead of meo. In English we say "in my hand", but the Romans did not think of it that way. They thought of it as "in the hand which is belonging to me" and so the Dative case is used in situations like this. So, for example, take this sentence from Sallust:
...neque mihi in manu fuit Iugurtha qualis foret (...and it was not in my power to shape the character of Jugurtha.)
So, we see in this sentence from Sallust a very similar kind of idea which you want to express and we see how Sallust uses the phrase mihi in manu, not in manu meo. This is the challenge of Latin: to learn the thinking behind expressions like this.
Now, before leaving this subject, I will throw one more idiom at you. In Latin, you can refer to your entire household and everything in it by using the word penates. Usually Penates means the household gods, what takes care of your stuff, but figuratively it can mean the household and its belongings and furniture and dependents inside of it. So, an alternative way to state your motto, if you are just talking about your household is:
Penates nostri mihi in manu (All my household is under my power.)