As Hikaria said you can't become immortal. Even if someone could find a way in the distant future, you would probably need a lot of energy to complete the spell.
The psychical body is the is the purest example of mortality, as it is organic (and the cells that reproduce slow down over time, ultimately becoming ineffective in keeping the body sustained).
However, I believe that the soul is immortal and you should focus more on the soul (my opinion and suggestion, you don't necessarily have to do this).
I agree with EisKora about the soul being immortal my personal belief is to become immortal first we must physically die, Then we are allowed to go home, personally myself i just think of being here as my soul learning lessons that I need to help me get to higher level in ''my real home''
The thought of being on earth forever is even worse a thought than burning in hell to me Lol.... that is just my opinion about what I believe.
But as for making a physically body immortal...I say this with tongue in cheek....you could always get your body cryogenically frozen but what become of your mind and soul got no idea. ;P
i know that this might be very scary and sound weird but this is also a way
Your soul is already immortal, but if you don't want to leave this earth then you should send you soul forcibly to another young dead body when your body gets old, also remember that you should move your soul to a young person who is dead
WARNING:this is too dangerous and has many risks, most of the time the dead body might reject your soul and you may ended up dead, you may forget your old memories,you may feel very uncomfortable with your new body, you soul might not be comfortable with the new body and so on... the above listed are very few and i suggest NOT TO DO THIS, but if you really have a reason then you can
do the right thing, being immortal is a curse not a boon