Just work with what you have. It's perfectly fine. At some point you will find it is holding you back. Then you can start shopping for an upgrade. You may find that point to be much sooner than you expect, but that depends on how fast you progress.
Let's take apart all the gobbledegook that identifies your lens.
75-300 1:4-5.6 III IS
75-300 is the range of focal lengths that this lens covers. Focal length is measured in mm. Technically it's the distance from the rear "nodal point" to the plane of the sensor (or film). In practical terms, it determine the field of view of the lens on a given camera. 75mm is zoomed all the way out. 300mm is zoomed all the way in.
1:4-5.6 OR 1:4-1:5.6 OR f/4-f/5.6
This is a range of fractions that are the maximum size of the aperture, or the opening in the lens. The "f" is a variable for the focal length. Let's look at a simpler lens to make this easier to understand.
50mm f/1.4
This is a prime lens, meaning it does not zoom. It has a fixed focal length of 50mm. It has an invariable MAXIMUM aperture of f/1.4. This does not mean that the aperture is always f/1.4. It means that the widest that the aperture can be set at is f/1.4. So what does f/1.4 mean? I mentioned the "f" is a variable (like algebra) for focal length. With this lens, the focal length is 50mm. So, the physical size of the aperture is 50mm/1.4. 50mm divided by 1.4 is ~36mm. This means that the maximum physical size of the opening in the lens that lets light through, is about 36 millimeters across (diameter). You do not need to know this, but it might help things "click".
So why express it as a fraction instead of just saying max diameter of 36mm? Because the aperture is dependent on the focal length of the lens! A 100mm at f/1.4 is ~71mm across. This is why a 100mm f/1.4 lens must be physically wider than a 50mm f/1.4. What about a 600mm f/4? 150mm across. That's why long lenses also must also be large lenses. And why the faster lenses (f/1.4 is faster than f/5.6) must be wider than slower lenses at the same focal length. There are other things inside of a lens housing besides just the aperture, so you can't say EVERY fast lens is larger than it's slower equivalent. Image stabilization takes up a lot of room, creating "fatter" lenses.
So why does your lens have a "range" of maximum apertures like f/4 through f/5.6? Bottom line, to keep the lens affordable and/or small. It's easy to do f/4 @ 75mm because the aperture only has to be ~18mm. But if we try to keep f/4 at 300mm, the aperture would have to be 75mm! That means larger pieces of glass, more expense, more weight. So the aperture is restricted to ~56mm (300mm/5.6) across which is significantly less.
So, as you zoom from 75mm to 300mm, the maximum aperture changes from f/4 to f/5.6 incrementally. On more expensive zoom lenses, the aperture changes linearly as you zoom in so you get a constant f/# like f/4 70-200. The actual maximum size of the aperture still changes as you zoom in, but the f/# stays the same.
So who gives a crap? How does any of this relate to me taking pictures?
The f/# is a constant amount of light. f/4 on a 100mm lets in the same quantity of light as f/4 on a 70-200. Only the physical aperture size changes with focal length in order to compensate. The amount of light stays the same. So, the higher the f/# (f/1.4 is higher than f/4 because it's a fraction, this is where terminology gets confusing), the more light is let in. Which means you can get faster shutter speeds. Which means less motion blur. Also, the wider your aperture, the narrower your depth of field becomes. This can be desireable, or not, depending on how you want the photo to look.
Finally, IS means image stabiliziation, but you probably already knew that.
And sorry for the book. I hope that is accurate and understandable. If I made any errors, someone please correct me.
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