I’d recommend starting with IKEA. They have amazing items at affordable prices. Collect photos or create a lookbook of the style you want of a particular room. Then visit IKEA and see if similar items are available. Also keep an eye out online to find related items which can be easily added to your apartment. Another idea is to build your own home decor items. You can build your own desk or dining table, or even create your own art and frame it. The options are endless.
I love visiting consignment shops, GoodWill, etc to find really cheap and unique items for my home. I visit my local one but I also to go neighboring ones (sometimes even in uber rich areas) to find amazing pieces for apartment that noone else has. You can also have a friend who has a good eye for design, give you suggestions on your decor and you can both do a DIY together to put in your space. Some interior designers also have cheap starter packages that can help guide you and get started.
Consider printing an old, vintage map. You can download them free from the Library of Congress website. Companyies like Ted's Vintage Maps digitally restore them and then sell them... but a lot of the maps don't need restoring... so your only expense would be getting it printed.