If you're enrolled in a college taking a GIS class, check to see that you don't already have access to the ESRI training website. With that course, you should have access to an ArcMap/ArcGIS Pro license so that you can use the software on your laptop, no point in paying $100 for something you may already have access to.
The ESRI website should have a lot of content that is included with your license... only a few of those are full on classes that cost $$$. Many of them should be free as far as the basics are concerned... they'll usually say "Maintenance Required" with an unlocked padlock symbol, those are the free classes you can take.
The trouble with learning this way is that it can be difficult to learn without a road map and end goal... especially as it pertains to tree inventories. I wouldn't discount your college course though just yet as the theory behind GIS is pretty important in understanding what all of the components are such as PCS vs GCS, raster and vector data, basic cartography principles, remote sensing, feature data storage formats, geodatabase design, and etc...
However, if things don't improve by the deadline to drop, look into "MOOCs" aka Massive Open Online Courses that are offered through websites such as Coursera, edx, and Udemy. They are usually much cheaper and a lot of them are offered by large universities or industry professionals, so you'd be getting hopefully decent instruction. Just go on one of those websites and search GIS and there should be a few introduction to GIS classes. For example,
here's one that is taught by someone at the University of Toronto (and it's apparently free!). The big upside to these courses is that many of them were designed from the start to be taught online.
Realistically, you can take one of those introductory courses and be competent enough to put together a tree inventory for someone or some organization... but that first project you undertake is going to be a giant PITA as there will be a lot of specific things you'll need to learn. So while you may understand what a file geodatabase is, how to create and edit features, use the field calculator, and make basic maps in the layout view... there's a whole other world of things you'll have to eventually start considering and incorporating into your projects to become more efficient, scalable, and profitable.