1. Always be jumping and spam crouching. Even the sweatiest of snipers will have a harder time headshoting you while you're doing that.

2. Try to stay out of the snipers sight line while still moving towards the sniper. This might seem obvious, but it can be a challenge against really good snipers.

3. Don't let the sniper get away. Snipers will often try to run from you as you approach them, typically towards a health pack. Try to cut snipers off as much as you can and back them into corners.

4. Use flank routes. Snipers often have extreme tunnel-vision, so they won't be able to focus on their main area and a flank route that often. Use flank routes to get as close to the sniper as possible.

The best class for dealing with snipers is demoman, as he has a variety of tools to get close and personal with snipers. Demoknight has trimping and standard demoman has the sticky jumper. Both are excellent options for dealing with snipers.

The second best class for dealing with snipers is scout, as with the highest base move speed in the game snipers have a hard time hitting them. Scouts can also get close much faster than other classes (except demo of course). For loadout options which primary you use doesn't effect things too much but for secondary and melee I would recommend bonk atomic punch and the atomizer. The atomizer will help you get to some of the higher common sniper spots while the bonk will enable you to temporarily move through sniper sight lines safely. You may want to be careful with the bonk as you can sometimes end up in a bad spot without the ability to run away so try to get to a safe area before your bonk runs out.

The third best class for dealing with snipers is soldier. Soldier can get close with his rocket jump, and most snipers will have a hard time running away from a soldier back to spawn. For loadout I would recommend either the gunboats or the rocket jumper. The gunboats can enable you to get to the sniper with only a minor amount of health lost and the rocket jumper can be paired with the market gardener to crit snipers into oblivion.

The fourth best class for dealing with snipers is spy. You may think that spy should be higher, but both the razorback and jarate prevent spy from being a sniper killing machine. Most snipers will switch to the razorback after being backstabbed more than twice, and they basically can't be killed with backstabs after that. Your revolver is another option but be aware of where the sniper's team is and if the sniper could kill you before you can kill them.

Some other classes that can be somewhat useful against snipers are fists of steel heavies, vaccinator medics, and thermal thruster pyros.

Sniper is theoretically an option for countering sniper but that's cowardly.

80% of all sniper mains have what is commonly referred to (by me) as sniper's syndrome. It turns ordinary people like you and me into toxic, sweaty psychopaths. Most snipers will attempt to get into your head, and make you ragequit. The most effective strategy against this is to simply mute them. Although I would highly recommend just muting them, if you are 110% sure you can take them on, then go for it. A small win for you is a big win for all the sniper haters out there. But as soon as you stop having fun in a match mute them immediately. Trust me on this one, I've seen it play out 100 times before.

While these are not all the ways to counter sniper, they are the ways that I have found most effective during my 1000+ hours of playing this game. This guide will probably not give you everything you need to counter snipers, as the most valuable tool against snipers is experience. This guide is simply a general guideline to get you started on your sniper killing rampage.
and yes I am an engi main how could you tell