Honestly, if you're playing top lane. Your main goal before any towers get taken is to get as much cs (creep score) as you can. You get cs whenever you kill a minion, and as you're aware, landing the last hit on a minion grants both gold and experience (you get experience regardless if you don't last it, but you don't get gold). Top lane champions are typically those focused around a mid to late game play style,
with few assassins being played top right now. As for when to roam down to help out your team, you can do a short roam to the middle of the river whenever you completely push a wave under tower (this means the tower is targeting not only the melee creeps but also the ranged creeps.) A roam farther than that, would be a waste of gold and experience unless you can guarantee a kill (such as on an immobile mid
who has already used flash
).
After taking your tower (or getting your tower taken) you basically just want to farm top when the wave is on your side of the map, and group for objectives (towers/dragons) when it isn't.
Depending on the champ you play, a viable strategy might be to just sit top, and try to get the enemy team to 2v1 you, with the intent that you will escape the 2v1 situation or kill someone in the process. By drawing 2 or more enemies to your lane, your team has the option to engage on a 4v3 scenario. This tactic is best employed with a strong duelist
while an objective (tower/baron/dragon) is able to be taken by your team. If they only send one champion up top to contest you, and you see each other enemy champion on the map close to mid/bot lane, you can engage on the opposing enemy and hopefully solo kill them if you are ahead.
Otherwise if you're looking to play team-fight champs (such as tanks or some bruisers)
then as above, push out lanes, ward jungle ramps (those little pathways leading to the river) and look to catch out enemy champions that are moving around the map by themselves.
In summary, get cs until a tower is lost (your's or their's)
Once it's gone, push till the wave leaves your side of the map (you can let your team know you're doing this by pinging them off of objectives or fights with the caution ping. Some objectives/Team fights take priority over cs tho, so make sure you pay attention to what objectives are up on the map.)
Group with team when no safe cs can be taken
You only want to roam from top if you can guarantee a kill, otherwise, as you've noticed, you'll arrive too late and get nothing while losing cs from your lane.
(note there are many more champions than the ones I listed above, that fit specific roles.)