Can i use my apple id to my iphone 5s

hi Markchristianmangubat,

yes, you can.

Take your iPhone that you want to access.

1. Launch the Settings app on your iPhone or iPad.
2. Tap "Sign in to your iPhone" (or iPad).
3. Tap Email and enter the email address associated with the different Apple ID.
4. Enter the Password associated with the different Apple ID.
5. Tap Merge if you want to merge the data on your iPhone with the iCloud account associated with the different Apple ID. Tap Don't Merge if you don't want the content on the iPhone uploaded to iCloud in the different Apple ID's account.

See here how to Sign in with your Apple ID - Apple Support

Yes, you can. Make a full backup of you old iphone, then follow all the steps provided in this support guide:

What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support

Regards

Giulio

Well, this is a totally different problem.

If that iphone has activation lock, only the previous owner can unlock it (following the steps provided in the support guide I already suggested):

What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support

Nobody else can help you here. If the previous owner doesn't follow all the above steps, that iphone is useless for you, sorry.

Regards

Giulio