Here's a little tutorial
The game works like papers please. You don't actually have to do anything sexual with the girl to gain points. When interviewing a girl, look at the requirements on the top right, they'll be things like "has long socks" or "has white panties". If you see an incongruity, click on it (on the girl) and then immediately click on the red triangle button on the right. You'll hear a little sound affect if you did it right. With every successful interview you'll get money, progression points, and skill points at the end of the session.
On the main screen where you select the stages, on the top left corner, you'll see the requirements for progressing the stages. Some of them will require you buy items from the shop. Stage 3 requires the "med kit" for injured characters and stage 4 requires the "report slip" for spies both of which will be available in the shop once it's required to progress.
The tabs on the left on the main menu are pretty self-explanatory: stage select, saved characters, shop, skill shop, and achievements. I believe all the values are 4-byte on cheat engine if you want to quickly increase a value or just use the debug menu that one of the commentors above mentioned.
Feel free to ask if you have any more questions.
