One of the best things you can do will only cost you about a bag of charcoal and a small box of foil:
First, most budget horizontals leak like a funnel. Take foil and roll out several "snakes" about 1/2" in diameter. Then, mash the snakes all around any door opening; both in the smoke chamber and the fire box. This will seal the box allowing for more temperature control by evening out the draw; which I talk about below.
Load your firebox up and dump a chimney of lit coals on top. Open your exhaust fully and set the intake damper to about 1/3 open. Let that run for 30-45 minutes to settle in. Take a temperature measurement in the center of the smoke chamber at grate level (it's best to use a digital remote so you don't have to open the lid. Whatever you use, don't trust the one that came with your smoker. It lies!). This is your "sweet spot" temp. Is it lower than you want (depending on what you are cooking)? Open it to about half and let it run again for 30 minutes. Check again and see how much it changed. At this point, don't constantly adjust your damper to "chase" a temperature. Make changes and wait. See where the temperatures settle given the location of the damper. This is why you don't have food in your cooker at this point. You are focusing on temperature control, not dinner. Use up the whole bag until you start to get comfortable with where your smoker want's to land.
If your boxes are sealed well, and cooking conditions are fair (not terribly windy and no snot cicles) future cooks should be relevant. Temperature control is all about how much air the exhaust stack can suck air through the firebox intake and into your fuel source. This is known as "draw". You could close the exhaust to reduce draw, but that causes stagnant smoke which can lead to bitter flavors. Instead, control the draw by the amount of intake opening.
Also, think of temperature control adjustments like driving. You don't come to a stop sign at 60 MPH and slam on your brakes, eh? No, you see the stop sign coming and slowly apply brake pressure. Think of the damper as a break pedal. When you see your desired temperature approaching, slowly make adjustments needed. When you are cooking don't wait until you are at the temperature and make a change, because you will overshoot. This is what causes most babysitting of smokers. If you are always overshooting, you will always be playing catch up. Get a feel for how long it takes for damper adjustments to change the temperature. Then, make sure you give yourself enough time before your desired temp is reach.