Is it a sin to have any kind of surgery like plastic surgery? Are tattoos sinful even if they don't represent anything bad?
Under the Old Law, the Israelites were forbidden to imitate the idolatrous religions around them. One practice was to make marks on the body, either by tattoos or by scaring to show which god (or ancestor) you served. So God told the Israelites, "You shall not make any cuttings in your flesh for the dead, nor tattoo any marks on you: I am the LORD" (Leviticus 19:28).
Today, tattoos are used mostly for decorative purposes, but they can still be used wrongly. To use them to make statements that should not be associated with Christians would be obviously wrong. "But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints; neither filthiness, nor foolish talking, nor coarse jesting, which are not fitting, but rather giving of thanks" (Ephesians 5:3-4). But we should also be careful about subtle associations. We don't want to tie ourselves to organizations or people who later do not represent proper Christian attitudes. "Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness?" (II Corinthians 6:14). While vows of fellowship may not be involved, still a tattoo is basically a permanent mark and people will make conclusions when they see it.
Plastic surgery is most often used to correct damage from injuries or diseases. For example, burns or skin cancer. Some people use it to remove the marks of aging. For the most part, this latter group is motivated by vanity or pride. This is contrary to the way Christians ought to be. But you can't condemn the entire practice when it is only used poorly by a few people.
Basically apply the same principles of modesty used for selecting clothing (I Timothy 2:9-10). You don't want to call attention to yourself because of a tattoo you are wearing or the surgery you had done. If people notice you, it should be because of your Christ-like behavior.
So the sin is to show off, or is it a strong desire for attention?
What if you bought a really nice shirt or something and you wore it on a special occasion, so people can notice it. Is that considered sinful?
What exactly is vanity? For example, is lipo-suction sinful if someone is trying to look better? I also noticed that some guys like showing off their bodies when they go to the beach. Is that vanity?
Vanity is literally the idea of doing something that is worthless or non-productive, such as trying to hold oil in one hand or blowing into the wind to change its course. Striving after beauty is vanity because beauty doesn't last. "Charm is deceitful and beauty is passing, but a woman who fears the LORD, she shall be praised" (Proverbs 31:30). We just don't live all that long and we fall apart the longer we live. "For He knows our frame; He remembers that we are dust. As for man, his days are like grass; as a flower of the field, so he flourishes. For the wind passes over it, and it is gone, and its place remembers it no more" (Psalm 103:14-16).
It isn't wrong to look good, but few people stop there. Pride gets involved and looking good, or at least better than most people, becomes a high priority with them. The emphasis is all wrong. Instead of seeking after enduring qualities, such as your character, people only think about fleeting things. "Do not let your adornment be merely outward -- arranging the hair, wearing gold, or putting on fine apparel -- rather let it be the hidden person of the heart, with the incorruptible beauty of a gentle and quiet spirit, which is very precious in the sight of God" (I Peter 3:3-4). Looking nice on special occasions, such as at a wedding, is a way of saying "This event is important to me." But when a person wants to be the center of attraction, he is in for trouble.
The problem is that misplaced priorities get people into trouble. Guys who show off their bodies to get girls to notice them end up with girls who are only interested in their physical looks. Since character isn't important, the relationship is shallow. Disagreements break out. Someone more handsome, or at least new, comes along and she's off with the new hunk. "Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. For all that is in the world -- the lust of the flesh, the lust of the eyes, and the pride of life -- is not of the Father but is of the world. And the world is passing away, and the lust of it; but he who does the will of God abides forever" (I John 2:15-17).
Showing off is a form of pride and striving after popularity. The problem is that people are fickle. You can be popular today and a has-been tomorrow. "For he comes out of prison to be king, although he was born poor in his kingdom. I saw all the living who walk under the sun; they were with the second youth who stands in his place. There was no end of all the people over whom he was made king; yet those who come afterward will not rejoice in him. Surely this also is vanity and grasping for the wind" (Ecclesiastes 4:14-16). Or as it says in Proverbs, "A man's pride will bring him low, but the humble in spirit will retain honor" (Proverbs 29:23).
When we let pride run our life, we lose God. "Therefore He says: 'God resists the proud, But gives grace to the humble'" (James 4:6).