That's going to take some work! Can you share a photo of what you have to start with?
Lisa, it isn't going to be easy answering without a photo or at least a complete description of what you are working with and your budget. Please let us know more so we can help you!
Depends on how high that rack is... but can you slide a wood top through the top rung, and add 2 legs to the other side of the top, and still slide a chair underneath with you sitting on it?
This may end up costing more than a 2nd hand desk would cost at Salvation Army. Most magazine racks are not high enough to use as a desk. Besides thrift shops , check facebook market place, garage sales, Craigs list for used desks. Also ask around. A lot of people have a old desk sitting around, ready to get rid of.
Agree with everyone above, who has said that this project idea may not be cost effective.
There are numerous styles of magazine racks,......
If you posted a picture of yours we could see what you have to work with.