Plant and weed identification is often one of the hardest tasks asked of Master Gardener Volunteers. It's near impossible to know what each and every plant may be, but through practice you can develop a familiarity of the plants around you, and learn how to properly identify plants you don't know.
Plant identification works best if you have a complete plant sample, or several pictures with identifying information. Several leaves, a flower, and picture of the trunk will be far more helpful identifying a tree than a leaf found on the ground. Use these to determine what characteristics make your plant special.
Using the plant characteristics you've determined, utilize field guides that are most able to help you identify your plant, such as Trees of Minnesota or Wildflowers of Minnesota by Stan Tekiela. Field guides walk you through the identification process, and are beneficial for helping you compare potential matches.
Dichotomous keys offer the ability to identify plants through a series of choices, using the characteristics described above. Working through dichotomous keys can be labor intensive, but effective with the right key.
There are several phone apps for plant ID. Most rely on user generated identification or computer algorithms, which are not always accurate. Always double-check the suggested identification and compare with your sample before confirming proper identification.