start by browsing the site (especially the tutorial) and the forum.
Then use the tools provided in the member's section and base yourself on the recommendations of the different strategies, especially to start with the Global Dividend Growers.
how much to start with... let's say you follow the rule of no more than 1% in transaction fees, so I'll let you do the math based on your broker's fees in relation to your first purchase
DEGIRO I don't know at all, not known in Switzerland.
price as such is meaningless
a share at USD 15 is no cheaper than one at USD 30
always compare the price with a benchmark: profit, dividend, book value...