Hi Nanako! There are great answers for you people have already provided. I thought I'd add my two cents! (That means provide some input.)
Incline, when speaking about a hill or slope, generally means "going upwards." It can used with words like "steep: a steep incline; sharp: there was a sharp incline right before the pinnacle of our hike; slight: I prefer walking on trails with a slight incline so I can work my muscles in my calves and quads."
Tilt is generally used for either type of slope. You can tilt a surface at any angle. I can tilt my head and it simply means to move it at an angle. I can tilt the suitcase so all my clothes fall out.
I am inclined to believe in something is another usage.
Also incline vs. recline. I recline on a coach means I lie down on a couch. I recline in the easy chair. You see here the antonym for incline which has a sense of "getting up, going up"
Keep on learning!