The engineer class in Team Fortress 2 is one of the most versatile classes, although it doesn't seem like it. The engineer has two main load outs, or groups of weapons, and each has its own advantages and disadvantages. You can select the main wrench in order to build an up-gradable sentry up to Level Three, or you can select the Gunslinger in order to build a mini-sentry.
The advantage of a level three sentry is that it does more damage, and is perfect if you want to defend your team in their current position. Most often, you will see engineers using their level-three sentries to defend the goal point, such as on a Payload map, where BLU is on offense and their goal is to push the cart to the end goal point. It is possible to use this load out on offense as well, although it isn't commonly seen. With the right mouse button, you can move your sentry, even if its fully upgraded, to anywhere else on the map. This makes the level-three sentry extremely versatile and usable as an offensive weapon. Keep in mind, though, that while carrying your sentry you will be marked for death, which means that the enemy will receive mini-crits when they hit you. As an offensive level-three engineer, don't forget your other buildings. A teleporter in the right spot can get your team up to the front lines faster, and will also allow them to help defend your sentry. You can use the Rescue Ranger as well to repair and pick up your sentry from a safe place if it is under attack.
An Engineer update to introduced a wrench replacement called the Gunslinger to the engineer class, which allows engineers to place mini-sentries anywhere on the map for 100 metal. These sentries do less damage and are less durable than level-three sentries, but their fast building speed allows them to be placed and built fairly quickly, allowing your team some much-needed extra damage when playing offense. You can use the mini sentry in a number of ways, either for extra damage, or even to deter rocket-jumping soldiers from coming near your team. While you cannot repair the mini-sentry if it is being damaged, its low metal cost and fast building speed means you can set them down pretty fast while remaining mobile. You can combine the Gunslinger with the Wrangler in order to add a little more durability to your mini sentry, while also being able to pick your target better. The Wrangler allows you to control what targets your sentry is hitting, and adds a very useful shield to your sentry, but only when you are controlling it. This us useful in situations where a lesser threat target is closer to your sentry, drawing its fire, and a higher threat target is shooting it. The Wrangler allows you to be more involved as an engineer, and introducing a little more strategy to the engineer's play style. In Team Fortress 2, it is important to remember that every class can be useful. If you are having difficulty playing a certain class, all it usually takes is a little practice. Watch other engineers and learn how they play, then tweak your own play style to either mimic theirs or make it better.