Describe a plan to progressively ski harder trails. Recommend a ski resort in Tahoe including trails that I can use to follow the plan. 
 Skiing is a fun sport with something for every age and skill level. The measure of "hard" in skiing may be subjective but as a general rule of thumb, trails become harder as the steepness or pitch increases, number and type of obstacles increase, the trail is bumpier or is narrower. Changes along these dimensions present new challenges and require a broader array of skills. 

A good way to progress along trails is to pick a trail that presents a challenge in one or two dimensions of "hard" to learn or refine skills. As you progress, you can increase the challenges along different dimensions. Labelling at ski resorts help with these choices. Trails are generally classified into bunny (easiest or learning greens), green (circle), blue (square), black (diamond) and double black (two diamonds). When you are learning skiing, it's best to get reasonably confident at a color before moving on to the next one. You don't have to be perfect but you have to feel safe as you progress. While colors are broadly useful, they can still be inexact as measures of hardness. Further, your strengths and weaknesses will also play a part in determining how hard a trail is for you personally. Staff at ski resorts can point you to good trails based on your personal comfort. Remember to always ski with a friend !

For following this plan, you should consider Kirkwood Ski Resort near South Lake Tahoe. Kirkwood has a reputation for some great advanced skiing trails but it presents many good trails for progressive improvement. If this is your first time skiing, a lesson is strongly recommended to learn the basics of keeping yourselves and those around you safe ! 

You can start on the Timber Creek Area where there are a few bunny trails that are perfect for beginners. Funny bunny and School Run offer plenty of gentle practice for a new skier and when you are ready, ski Graduation (cue confetti). Squirrel Ridge and Snowkirk (on the Mountain Village side) are good next steps because they are long gentle greens.

Once you are feeling confident, Free 'N' Easy and Hole 'N' Wall offer a gentle introduction to blues. Blues at Kirkwood cater to a wide range of skills. The areas around Lower Zachary and Flying Carpet are good next challenges. If you are feeling good on those blues, try the backside with the appropriately named "Happiness Is"

If you've really got it down, it's time to try some black diamonds. Groomed blacks are easier to ski and fresh snow lowers the challenge further. Try Zachary and the trails around it. At this level, each black may offer a different kind of challenge. Hully Gully may have bumps and Lower Olympic is a nice introduction to a bowl.  There are many to choose from !

The final challenges are the double blacks and the The Wall is well known favorite. There are no dearth of double blacks and off piste trails at Kirkwood for those that have progressed beyond the labelling.

Have fun and stay safe !