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.
The cheapest way to decorate your place, especially if you are just starting out is to come up with a plan first. Preventing mistakes and having a clear plan will help you when shopping to not impulse buy and spend money on things that are not right for your space.
Even if you do it yourself make a notebook or a file of:
- Inspiration images that include a color palette, window treatments, and other design details you want to include.
- write your ideas down. This will help solidify them and help you if you have to go back to after a while and then you will remember what your thinking was.
- Take measurements. Make yourself a drawing of your floor plan and note all your measurements. You will refer to these often, trust me.
- Make a list of:
-- Things you need to buy and what size you are looking for. Example - 2 end table no smaller than 19" x 24"
-- Things you need to do to your structure. Example - add molding trim to the top of all doors and windows
If you need more help learning about how to plan your space you can look at this article https://www.mydesignlayers.com/designinginlayers/
Spending the money you do have wisely is the best way to save money on any project.