On getting a house
Being able to remove your rent and buy property first would be great but it really depends on personal circumstances.
If you already know where you want to settle, you may want to look into securing a house now. Keep in mind that mortgages tend to be quite expensive, based on your ability to repay and the country you're in.
I suggest you to run some calculations to see how much more you would end up paying a sample property with the best mortgage you can access.
If you think you'll have clearer ideas in a few years or if the mortgage is much more expensive than paying rent for the years it would take you to cover the full amount - you can rent and be more flexible for a while, while you save money to buy property.
On getting a house: conclusions
If you end up getting a mortgage, try to get a mortgage that will let you pay early without fees and to put all your money you earn in the mortgage.
If end up saving money for a property that you'll buy in less than 5 years I suggest you to put your money in saving accounts, even if they pay less, or slightly above the inflation rate.
The reasoning is: the more rewarding the profits, the higher the risk - the higher the risk, the more volatile the investment - the more volatile the investment, the longer you would need to wait before seeing some profits.
On investing your hard earned money
After having decided how much you want to invest
Given your age and your self proclaimed time frame of 15 years, your attitude to risk should be medium-high.
I would recommend investing in an index.
There is a lot to say about indexes. Summarising and simplifying a lot, they let you invest in a lot of companies from all over the world, diversifying your portfolio and therefore decreasing your risk.
More in particular I would recommend investing in a 100%/0% or 80%/20% stock/bond index fund with a global exposure(like this one).
The differences between bonds and stocks are another huge topic.
Summarising and simplifying a lot, Bonds are debt securities, stocks are ownership stakes.
The data we have shows that generally stocks beat bonds over long period of times (5-10 years) - even thought past performance is not indicative of future results.
Some people say bonds are less risky than stocks, other people say they aren't.
The reason for recommending a index tracking the world is, again, to increase diversification and reduce the risk of some economy collapsing and damaging your portfolio.
The market will probably keep on growing for the duration of our lifetimes.
On increasing significantly your net worth
Starting your own business and generating money passively is the only way to increase significantly your net worth (unless you're lucky or your salary is measured in millions).
I would recommend against startups, unless you've the right contacts and a team with the skills to raise money round after round and sell/market your product. The statistics on startups don't look great and having to grow so quickly puts a lot of stress on people and on the business. What could have been perfectly healthy small business can burn to the ground because of that.
The possible businesses you can start are unlimited - the following are my personal thoughts on the subject, given my personal circumstances:
Being in software myself, I think there is a good potential of making small passive revenue streams by building B2B small businesses.
The rationale behind going B2B is that you've lower marketing and sales costs, you don't need to do many small sales but a few big ones, businesses could be less inclined to switch to a competitor (because it's a cost for them), businesses could generate a smaller amount of support requests (just because of the smaller number of people).
Best of luck