The West Buttress route on Denali is not considered the most technical alpine climb, but it is a difficult mountaineering route where you need to have basic alpine climbing skills and excellent fitness. These can be learned on just about any introduction to mountaineering course. Don’t underestimate the challenge — you will be on steep terrain for hours, and your skills with crampons and ice axe self arrest, etc. are important for the safety of you and your team. You should be able to demonstrate a high degree of familiarity with basic mountaineering techniques such as the rest step, French Technique, front pointing, running belays, and roped glacier travel techniques.
You need to be in very good physical condition to climb Denali. Summit day shouldn’t be maxing out your strength and endurance, it should be well within your comfort zone. So many variables are out of your control (weather, etc.) and you need to be sure that you can manage the ones that you can control. Climbing big mountains is primarily an endurance event, but you do need strength as well. You’ll often leave base camp with 50+ pound (22+ kilogram) packs, and drag almost as much in a sled behind you. You should train with a pack as a part of your training regime so that your body can adapt to it.
Experience on a couple of other mountains prior to going to Denali will also help you understand where you are physically. Expeditions that you may want to consider before attempting a Denali climb are Mount Superior in the Wasatch Mountains and Aconcagua, the highest peak in S. America. This is a great chance to get expedition experience in high altitude in a much friendlier environment than Denali.
Note: Guides may decide not to take climbers up Denali if their skill level or fitness are deemed inappropriate during the climb.