Wow, well it depends on a huge variety of factors.
Any of the 2N series (ex. 2N2222 NPN, BJT) will work, but depending on the application and specs will require different current limiting, biasing, etc. The load (what is being switched on and off?) and the desired frequency, acceptable phase shift, power usage, etc will all come into play. Oscillators can be be varied with R-C components, crystals, etc. You'll need to decide if you want to go the FET or BJT transistor route, and you'll need to understand why...advantages, disadvantages.
Look, being that you are asking what is considered a fundamental question about a fundamental device, I can only assume that you have no idea what you are doing. To say the least, it would take writing a book to explain all you need to know to design this yourself. My suggestion is that you build something that is already a proven design. There are PLENTY, I mean PLENTYYYY of free schematics on the internet. All you need is a search engine and computer. The keywords I suggested will drop a million webpages on your lap. You will need to decide what method of wiring this up you favor. Breadboarding, board etching, or point to point construction. You will have to learn the proper method for these techniques.
I can only get you started by sending you to Google to find the circuit that is reputed to do exactly what you desire.
A hint: There are oscillator ICs (integrated circuits) that pretty much do it all within one component, (a chip) with having to only add a handful of external parts and the power supply....typically a few capacitors, resistors, some protection diodes, and possibly the output transistor(s). But you'll need to determine your needs and the load specifics even with this, choose the correct IC, and learn to read the datasheet to properly fit the external components...that is unless you find an exact circuit to fit your needs on the internet and simply build it per instructions.