Hohmann transfer
To move between two circular coplanar orbits, a Hohmann transfer uses an ellipse tangent to both. Two burns — one at departure, one at arrival — give the minimum two-impulse Δv for radius ratios below about 11.94. The transfer takes half the ellipse's period.
Plane changes are expensive
Changing inclination by Δi at speed v costs Δv = 2v·sin(Δi/2). At low Earth orbit speed, a 28.5° change costs about 3.8 km/s — as much as reaching geostationary altitude. Plane changes are therefore made where the spacecraft is slowest, often combined with the apogee burn.
Launch windows and gravity assists
Interplanetary Hohmann-like transfers require the target planet to arrive where the spacecraft does. For Earth to Mars, windows recur every synodic period of about 26 months. Gravity assists exchange momentum with a planet to change heliocentric speed and direction without propellant.