Hello!
First of all, those GFX’s are amazing for a few months of work. 
I have some tips and tricks/feedback for you!
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I highly suggest using rigs or bones. Most rigs and bones are free and are very simple to use depending on the program you’re using. Using rigs or bones really help with posing the subject and make it look natural. To me, posing really makes the artwork feel alive and gives the subject character. It’s what tells a story in a work of art, so improving it to make it seem more natural would improve a lot.
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Improving on the background would also help a lot as well. Backgrounds could also tell stories and play a huge part in artworks, so picking the right one for your subject/render would help.
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Lighting plays a big role in artworks as well. Typically, I choose to draw on my own highlights and shadows instead of using lights within the rendering software. It allows me to better match the subject within the background and the surrounding area. Matching the subject with the background could easily be achieved using a hue/saturation layer or exposure layers. Highlights and shadows could also be achieved using exposure layers and a hue/saturation layer that matches the color of any background lights or any sources emitting light.
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For text, you want to find at least 2 good fonts that fit together. I think you did that quite well in your pieces. However, I think the positioning for the text should be changed. Moving them towards the bottom would be a good choice, as it’s not blocking her face or any accessories she has on, and it’s not blocking much except for her legs.
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Adding some effects to your artwork could add more depth to the final piece. A vignette is common but very simple and adds a nice effect. Some other effects could be added in, such as snow or rain, depending on your background. Particles, extra lights, fog/smoke, and other effects could also help improve. It is very easy and common to overload your artwork with effects; however, if you plan to add effects, try to go light, or else the artwork looks full, and your eyes won’t know where to focus.
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I also very much agree with @xleavh; studying and recreating multiple works from favorite graphic designers is a great way for practice and improvement. I’ve done so myself for the past 4 years creating GFX, and it really does help. I normally pick out the small tricks they use in their artwork and combine them to create an interesting and unique style.
Those are some of the tips and tricks I suggest using! I think overall, your artworks aren’t bad, but there definitely is room for improvement.
I wish you the best of luck in the future for graphic design! 
I suggest paint.net. It’s a pretty good program and is like a free version of Photoshop. (With a bit fewer features.) There also is a student plan for Photoshop, $20 for every Adobe app. If you still can’t afford that, then paint.net is a simple and easy program to use. Some editing websites are basically free versions of Photoshop but on a website, such as Photopea.
