OK .... here goes:
Assuming this is a brand-new-just-out-of-the-box & just-hung-on-the-wall-combi, the terminals which are inside it will have links across them which mean it will be quite happy to work WITHOUT either a timer or a room thermostat.
ALL it will need to make it work is a supply of mains electricity.
This will be supplied by means of a bit of 3 core flex which will EITHER be already fitted by the boiler manufacturer, or you will have to supply & fit to the terminals marked L, N & E inside the boiler.
The other end you can connect to EITHER a Switched Fused Spur - double pole with a 3 amp fuse OR a Pug Top fitted with a 3 amp fuse.
Now, if a timer is required, EITHER fit an internal one - supplied as an optional extra by the boiler manufacturer - which is literally plugged into the boiler fascia, OR fir an external one which is screwed to the wall & wired to the appropriate teminals inside the boiler - you'll need something like a 5 core flex or a 3 core + 2 core or 2 buts of 3 core - wired into the boiler where the link is which you will remove.
For the Room Thermostat, you'll fit this to the wall and wire it back to the boiler with either a bit of 3 core & earth if the Room Thermostat is fitted with an anticipator & needs a neutral, or a bit of 2 core & earth if it isn't fitted with an anticipator & therefore doesn't need an earth. the other end of this cable is wired into the boiler where the link is which you will remove.
If you use a Wireless Room Thermostat, the receiver for this is fitted to the wall next to the boiler and will be wired to the boiler as per the wired Room Thermostat - as above - but with either 5 core flex or 2 bits of 3 core.
Read the boiler manufacturer's instructions carefully & study both the wiring diagrams in those instructions and those supplied with the timer / thermostat.
I hope this helps.