As soon as you feel comfortable with SC, Jumps, Navigation and you have a decent jump range distance (15LY or greater imo) etc, try going to one of the rare route starports, I have posted some on the forums here. the one I normally do however takes about an hour to an 1 hr 30. I will list it here and in main forums.
I start in Coquim (but you can reverse the route and start in Lave/Leesti/Orrere also)
Step 1: In Coquim (Hiryama Installation), you buy Coquim Spongiforms (20 max) (all rares are bright yellow colour on commodities market, u can only buy a maxnumber which I will write next to each port, once u have the max in hold you can get no more, you will have to wait approx. 10 mins for respawn of rares in most stations to ensure you get the full amount).
Step 2: Plot course to Orrere (145 LY approx. and u will need a fuel scoop to refuel on your way), In Orrere (Sharon Lee Free Market) sell the coquim spongiforms (300k approx. for 20) and buy Orrere vicious Brew (16 max).
Step 3: Jump to Diso (Shifnalport) 7LY, and buy Diso Corn (15max).
Step 4: Jump to Leesti (George lucas) 3 LY and buy Azure Milk (7 max) and Lesstian Evil Juice (14 max)
Step 5: Jump to Lave (Lave Station) 4 LY and buy Lavian Brandy (7 Max)
Step 6: Jump to Zaonce (Ridley Scott) 5 LY and buy Leather Egg (1 Max, you can only get 1 so don't hang around)
Step 7: Plot Course for Coquim ( Hirayama installation) and return there, sell all the rares you have and the entire trip should net you in the region of 1.1 mill to 1.3 million credits. Repeat as often as you like or until you get brain freeze lol.
There are longer routes and more profitable but what I like about this one is it is staright forward with only 2 bigger than 100 LY routes (Coquim to Orrere and return). Around the Orrere region you have 4 other rare systems all within 1 jump distance. If you start with this route you can expand it as you feel more confident. There are some anarchy systems (Orrere) so you will be interdicted once or twice and be careful when fuel scooping but it is a relatively easy route with good CR for 1 hr 30 mins work, may take a bit longer when you first do it until you become fast at fuelling etc etc.
I will post this on main forum also for any new players that want to make some early credits. Best ship to do it in I find is the asp (84 cargo), you will need a ship with beteen 60 and 90 cargo space and a reasonable jump range (15 or greater I would recommend). The cobra or adder could do this I think if upgraded.
Hope this helps.