It'd take a book to answer your question in the depth it deserves. WWI represents a major turning point in how militaries used (and thought about aircraft). At the outbreak of the war, aircraft were used mostly as scouts and flying messenger boys. In the United States, for example, the Army's few aricraft all belonged to the Signal Corps!

Although initially regarded as "toys," by old-school military officers, scout aircraft quickly proved their worth. The Western and Eastern Fronts of WWI started off as swirling, mobile campaigns as armies rapidly marched into battle. On the ground, horse-mounted cavalry tried to sniff out the enemy. Above their heads, airmen put a modern spin on the scouting mission. In August 1914, one French spotter plane noticed a gap between two advancing German forces. Allied troops counter-attacked, halting the German advance and arguably saving Paris. But for one lone plane, the Great War mighy have ended very differently.

As the mobile warfare of mid-1914 gave way to trench warfare in late 1914 and 1915, aircraft became even more valuable. Cavalry, the armies' traditional scouts, couldn't penetrate the trench lines of the Western Front (the Eastern Front is still a more fluid affair at this point. Interestingly, future ace Manfred von Richtofen was a cavalry officer here at this point in the war).

Airplanes, and to some extent, observation balloons, could roam wherever they pleased. Two-man spotter planes soon combed the skies over the Western Front. Observers sketched maps and even used primitive cameras to take aerial reconnaissance photos. The intelligence they gleaned was vital. Before offensives, it helped planners discover enemy positions, particularly hidden artillery batteries. Those guns could savage attacking troops in no-man's land, so knocking them out with counterbattery fire was essential for a large-scale attack to succeed. For defenders, recon planes could spot enemy troops and supplies massing for an offensive, giving the defenders time to prepare a response.

Since one scout aircraft could indirectly do immense damage, more and more effort went into shooting down enemy scouts. Early air-to-air combat had an air of enthusiastic amateurishness to it. Pilots and observers brought aloft pistols, hunting vehicles, shotguns, and even a few machine guns. The tactics weren't terribly complex: Fly alongside your target and blaze away.

Eventually, people on both sides of the war started seriously thinking about how to effectively arm an aircraft. The initial Allied solutions are crude, but fairly effective. Some fit a Lewis gun to the top wing of a biplane so it fires over the top of the propeller arc. It's hard to aim, but its better than nothing. French mechanics fit heavy metal deflectorsi to the propellers of a few planes. It's an awkward solution, to say the least, but it's enough for pilots like Roland Garros to start rackin up a few kills.

Robert Shaw's book has the best publicly-available breakdown of Air Combat Maneuvering. It's very much worth a read. https://www.amazon.com/Fighter-Combat-Maneuvering-Robert-Shaw/dp/0870210599